Characteristics of coal-measure source rock and gas accumulation belts in marine-continental transitional facies fault basins: A case study of the Oligocene deposits in the Qiongdongnan Basin located in the northern region of the South China Sea

Abstract

This study carried out the development and distribution prediction research on coal-formed gas accumulation belts from the perspectives of the development and distribution of the coal measure source rock and the thermal evolution degrees of those rock masses in marine-continental transitional facies fault basins. The purpose of the current study was to improve the accuracy of gas reservoir explorations in the sea areas. The methods used in this study were based on the systematic analyses of the drilling, logging, geochemistry, and seismic data of the Oligocene deposits in the Qiongdongnan Basin of the northern region of the South China Sea. It had been previously established that the sedimentary processes of the fault basin were mainly controlled by faulting, and (fan) deltas had often developed in the fault bending sections, tectonic transformation sections, and fault ends. The distribution along the fault zone was observed to be beaded. During the early Oligocene period of the Qiongdongnan Basin, there were strong extensional actions, and the marine and continental transitional facies strata had developed forming the northern fault depression and the central fault depression. Three (fan) delta belts were formed in the slope belt in the northern fault depression and also in the slope belts located in the north and south of the central fault depression. Due to the superposition of the humid and hot climate during that period, the majority of the coal measure source rocks were developed in each (fan) delta. Also, three macroscopic coal measure source rock belts were formed. The types of source rock had mainly included autochthonous coal, allochthonous coal, and terrigenous marine mudstone, with terrigenous higher plant organic matter accounting for more than 50%. After the deposition of the lower Oligocene period, the basin continued to sink, and the coal measure source rock masses continued to be heated. During that period, the maturity of the organic matter had continued to rise. In the northern slope belt of the northern fault depression, the maturity of the organic matter in the coal measure source rock belt were low and the gas generation abilities were weak, which was advantageous to the formation of potential gas accumulation belts. The coal measure source rock belt on the northern side of the central fault depression had a high maturity of organic matter and strong gas generation ability, forming a macroscopic gas accumulation belt. In addition, the Yacheng 13–1 large coal-formed gas field had been discovered. The maturity of the organic matter had varied greatly in the coal measure source rock belt on the southern side of the central fault depression. The coal measure source rock masses with high maturity had strong gas generation abilities and had also formed a macroscopic gas accumulation belt, and three large and medium-sized coal-formed gas fields (such as Lingshui 17–2) had been discovered. The gas accumulation belts on the southern and northern sides of the central fault depression were favorable areas for coal-formed gas explorations. Furthermore, the trap zones in and near the coal measure source rock belts with mature thermal evolution were determined to be the most favorable areas for coal-formed gas explorations.

Keywords

Marine-continental transitional facies fault basin coal measure source rock belts coal-formed gas gas accumulation belts favorable area forecasts

Introduction

Coal-formed gas is an important part of natural gas and is the product of sedimentary organic matter in coal measure source rock masses at a high maturity stage (Dai et al., 2014). Coal measure source rock can be formed in various types of basins and are known to be widely developed and distributed. A great deal of research has been previously completed regarding the development and distribution laws of coal measure source rock in continental fault basins (Li et al., 2018; Liu et al., 2018; Lv et al., 2017; Shao et al., 2008; Li, 1998; Sun et al., 1998; Wang et al., 2019). The coal-measure source rock masses in marine-continental transitional facies fault basins are known to be relatively developed (Li et al., 2010, 2018), and the contributions of the coal measure source rock to coal-formed gas resources in those types of basins have been confirmed to be particularly large, especially in sea basins (Zhang et al., 2012a). For example, in the Oligocene marine-continental transitional facies fault basin, which is located in the northern region of the South China Sea, it has been determined that the coal-measure source rock masses are characterized by large thicknesses, wide distributions, small coal seam thicknesses, poor regional stability, and so on (Li et al., 2010, 2012). Moreover, the coal-measure source rock in the basin is known to be major contributors to natural gas reservoirs (Chang et al., 2013; Fu et al., 2010; Li et al., 2011a; Pandey et al., 2018; Singh et al., 2015, 2017a, 2017b; Wang et al., 2016). On a global scale, marine-continental transitional facies fault basins are very common, especially in the western Pacific Ocean post-arc region and southeastern Asia. However, little research has been conducted regarding the development, distribution, thermal maturity, and hydrocarbon generation characteristics of fault basins. Furthermore, the relationship between the source rock masses and oil and gas accumulations and the distribution laws of gas accumulation belts in coal-measure source rock masses within fault basins have yet to be explored. This situation has resulted in major restrictions in the exploration and development of coal-formed gases in these types of basins.

The Qiongdongnan Basin is located in the western part of the northern continental margin of the South China Sea and is a passive continental marginal basin which was formed during the Paleogene-Quaternary period. It has been determined that during the Eocene-Oligocene period, the basin was in a rifting stage. Also, during the Neogene-Quaternary period, the basin was in a depression stage, and during the Early Oligocene period, the basin mainly contained marine-continental transitional facies. It has also been confirmed that from the Late Oligocene to Quaternary period, the basin had mainly received marine deposits (Zhang et al., 2007).

The oil and gas explorations in the Qiongdongnan Basin have a long history. In 1983, the largest gas field in China (Yacheng 13–1) was discovered in the Yacheng Ridge area in the western part of the Ya’nan Sag, which is located in the northwestern region of the basin (Deng and Chen, 1989). In 2014, the Lingshui 17–2 gas field was discovered in the southeastern section of the Lingshui Sag, which is located in the southwestern region of the basin (Zhang et al., 2016). It was discovered through the correlation between the natural gas and source rock in the aforementioned gas fields that the natural gas in those large gas fields was mainly sourced from the Early Oligocene coal-measure source rock masses (Fu et al., 2010; Li et al., 2011a; Wang et al., 2014a). Several of the wells in the basin encountered coal seams which were formed during the Early Oligocene period, including the Y15a, Y16a, and L21 coal seams, and so on. In summary, the Qiongdongnan Basin has been found to be an ideal basin for studying coal-measure source rock belts and gas accumulation belts. In the current study, from the two aspects of coal-measure source rock development and thermal evolution, and in accordance with the “source-heat” co-control theory, the development of gas accumulation belts in the Qiongdongnan Basin and their distribution predictions was examined. It was hoped that the research results of this study would provide some guidance for oil and gas explorations in similar basins.

Tectonic framework and evolution

Tectonic framework

The Qiongdongnan Basin is a secondary tectonic unit located in the northern section of the South China Sea. On a macro level, the basin can be divided into four secondary tectonic units as follows: the northern fault depression; northern uplift; central fault depression; and southern uplift. It can be further divided into more than 20 tertiary tectonic units. The northern fault depression consists of three sags: Yabei, Songxi, and Songdong. The central fault depression includes six sags: Ledong, Ya’nan, Lingshui, Songnan, Baodao, and Beijiao (Figure 1). Longitudinally speaking, the basin has the characteristics of a superposed fault-depression with a dual-layer structure. The lower layer is an alternating concave and convex graben-semi-graben structure, and the upper layer is the depression structure (Li, 2004).

Figure 1.

Tectonic framework and gas field distribution of Qiongdongnan Basin (revised according to Li (2004)).

Tectonic evolution

When the Early Oligocene entered a fault period, the Qiongdongnan Basin was affected by regional differential extensions and block tilting-up activities, during which time such tectonic units as the northern rift and central rift zones were formed. Since the basin basement was an orogenic fold belt rather than a rigid belt, the newly developed fault depressions were affected by early faults in the basement region. Also, the central fault depression of the basin had become highly active and complex graben sags had subsequently formed in the central basin and semi-graben sag areas within the southern and northern margins of the basin (Zhang et al., 2015).

When the Early Miocene was in a recession period, the Qiongdongnan Basin was in a depression period, in which the regional extensions had gradually decreased. During this period, the basin had featured no fault activities, small subsidence amplitude, and only slight lateral differences overall. During the early depression stage, the subsidence had slightly increased and the basin had displayed a structural pattern of shallow depressions and low uplift. The strata in the central fault depression were evidently thicker than those in the northern region. In fact, the northern fault depression no longer existed, showing a monoclinic tilt to the center (Zhang et al., 2015).

The Late Miocene-Quaternary period was a new tectonic period, and in the Qiongdongnan Basin, strong plastic extensions had occurred in the central fault depression, particularly in the southwestern part of the basin. The tectonic subsidence was superimposed in the basin with sedimentary loading due to the combined substantial input of terrestrial debris carried by the Red River system, Hainan Island sources, ancient Pearl River water system, and so on (Shao et al., 2013). This had resulted in a “stepped” settlement of the western segment of the central fault depression in the Qiongdongnan Basin and the formations of sediment deposits with huge thicknesses. During this period, the basin had taken on a unified depression structure with the central fault depression as the axis (Zhang et al., 2015).

Stratigraphic development

The Paleogene and Neogene strata in the Qiongdongnan Basin are known to be relatively well developed. These include the Eocene, Oligocene Yacheng, and Lingshui, Miocene Sanya, Meishan, Huangliu, and Paleocene Yinggehai Formations. Generally speaking, the coal-bearing strata were mainly developed during the Oligocene period. The Lower Oligocene Yacheng Formation is a set of coal-bearing (fan) delta-shore shallow sea sediment, and the Upper Oligocene Lingshui Formation is a set of shallow sea-fan delta deposits with slight coal seams, as detailed in Figure 2.

Figure 2.

Paleocene composite columnar section of Qiongdongnan basin (by Li et al. (2011c)).

Ternary composition and distribution characteristics of the coal series source rock belts

It has been observed that there are terrestrial organic matter systems which correspond to each coal measure source rock belt of the Oligocene in the Qiongdongnan Basin. These systems are composed of three parts: allochthonous coal, autochthonous coal, and terrigenous marine mudstone. Furthermore, the organic matter of the terrestrial higher plants account for more than 50% (Figure 3), namely the ternary structures of the coal measure source rock masses.

Figure 3.

Distribution of autochthonous coal, allochthonous coal, and terrigenous marine mudstone in the (fan) delta area.

Previously, through examinations of the Yacheng Ridge deltas with large coal-formed gas fields, more drilled wells, and high degrees of research, it was revealed that the coal seams and developed delta environments contained higher total organic carbon (TOC) values, Type III organic matter, and greater hydrocarbon generation potentials. Moreover, the hydrocarbon generation parent material in the source rock masses were determined to be mainly from higher plants of terrestrial provenance (Cai et al., 2003; Li et al., 2011a), as detailed in Table 1.

Table 1.

Statistics on geochemical characteristics of Oligocene coal measures source rocks in the Ya’nan Sag.

Stara	Facies	Lithology	TOC (%)			S₁ + S₂ (kg/t)			Chloroform asphalt "A" (%)			HC,10⁻⁶			Kerogen type
Stara	Facies	Lithology	Min	Max	AVG (n)	Min	Max	AVG (n)	Min	Max	AVG (n)	Min	Max	AVG (n)	Kerogen type
Yacheng Formation	Alluvial fan	Mudstone	0.21	4.38	0.93 (22)	0.04	4.74	0.89 (23)	0.0028	0.0028	0.0028 (1)				III
	(Fan) delta	Mudstone	0.17	4.65	0.99 (79)	0.03	3.95	0.98 (48)	0.0009	0.1833	0.0601 (35)	129	843	345 (29)
	(Fan) delta	Coal	95.93	95.93	95.93 (1)	91.66	99.98	95.82 (1)	1.4	1.4	1.4 (1)	5600	5600	5600 (1)	III
	Tidal flat-Alluvial fan	Mudstone	0.15	1.47	0.48 (16)	0.01	1.18	0.24 (25)	0.0004	0.0679	0.0172 (24)				III
	Tidal flat	Mudstone	0.16	5.01	1.56 (56)	001	8.81	2.34 (67)	0.0035	0.6782	0.0876 (45)	106	3350	1179 (29)	III
		Coal	20.44	81.3	42.62 (13)	0.36	155.4	87.56 (12)	0.1908	2.995	1.6802 (5)	1049	12090	7415 (5)	III
		Carboniferous mudstone	6.39	19.94	12.96 (5)	7.6	49.32	22.51 (5)	0.3011	0.3011	0.3011 (1)	1626	1626	1626 (1)	III

Note: Testing standard: TOC, determination of total organic carbon in sedimentary rock (GB/T 19145–2003); chloroform bitumen “A,” determination of bitumen from rocks by chloroform extraction (SY/T 5118–1995); S₁ + S₂, rDDTock pyrolysis analysis (SY/T 5117–1996).

Autochthonous and allochthonous coal deposits

Recently, in the Oligocene Yacheng Formation of the Qiongdongnan Basin, a number of exploration wells have encountered coal seams. The coal seams were found to have small thicknesses, ranging from several centimeters to tens of centimeters. However, these were observed to be large in number and up to dozens of layers at most (Li et al., 2010). The most favorable facies belts for coal-measure source rock in the Yacheng Formation have been determined to mainly be the peat swamp areas of the (fan) delta plains; upper and intertidal zones of the tidal flats; and the coastal plains and areas behind the chenier (beach ridge tape tidal bank), which are distributed in the margin of the sags and low uplifts (Figure 3). Therefore, based on the distributions of the coal-forming sedimentary facies, typical coal-forming sedimentary models, and wave impedance inversions of the seismic data (Cai et al., 2013; Li et al., 2010), the distribution range of the coal seams (groups) in the Yancheng Formation of the Qiongdongnan Basin has been successfully predicted. It has been that the coal seams are mainly distributed within the northern fault depression, southern uplift areas, and the low uplift areas of the Qiongdongnan Basin as well as the northern and southern sides of the central fault depression.

In the areas where the coal seams have developed in the sag, there have been obvious differences observed, and regularity in the numbers, thicknesses, structures, and morphology of the coal beds in different parts of the sag has been confirmed to be due to the influences of such geological conditions as paleo-geographical and paleo-tectonic activities. The characteristics indicate that the development of the coal seams along the strata had obvious zonal tendencies, such as coal seam tip belts, coal seam merger belts, and coal seam bifurcation-tip belts as well as no obvious coal belts. For example, in the Yacheng Ridge area of the Ya’nan Sag, the coal seams have been observed to display obvious coal seam zones along the strata and have mainly formed in the delta plains. The coal seams in the transition zone near the land and offshore areas on the delta plains are the most highly developed, and the coal seams toward the land and sea areas gradually become thinner and sharper (Figure 4) (Li et al., 2010, 2012). However, it has been found that the terrigenous marine mudstone may extend farther, at potentially dozens of kilometers into the sea (Cai et al., 2003; Peters et al., 2000; Tesi et al., 2008).

Figure 4.

The tendency and zonality of the development of the coal seam in the third section of Yacheng 13–1 area west of Ya’nan Sag.

In the Yacheng Ridge area located in the western part of the Ya’nan Sag, the coal seams have mainly developed in the deltaic plain swampy environment of the third section of the Yacheng Formation, and the majority of the coal has accumulated in situ by coal-forming plants (Li et al., 2010). For example, the explorations of Well Y15a and Well Y16a have revealed that the organic maceral of the Yacheng Formation coal is mainly composed of vitrinite, with an average content of more than 90%. This is followed by inertinite and basically no liptinite have been observed (Table 2). The micro-litho types are dominated by vitrite (with a vitrinite content of more than 95%), which is followed by vitrinertite, which indicate that the peat was deposited in a shallow water-semi-covered environment. Therefore, the above-mentioned organic material, together with the generally high content of ash (exceed 33%) (Table 2), indicates that the coal seams in the Yacheng Ridge have the characteristics of hypautochthonous coal. That is to say, the coal-forming material or peat may have been transported over a short distance, but the coal seams still belong in the category of autochthonous coal (Liao et al., 1995; Zhang et al., 2010).

Table 2.

Types and contents of maceral of Oligocene coal in Qiongdongnan Basin.

Well	Stara	Organic macerals (%)			Inorganic macerals (%)
Well	Stara	Vitrinite	Inertinite	Liptinite	Clay rock	Carbonate	Sulfide	Oxide
Y15a	Lingshui Formation	89.7	2.6		6.9		0.4	0.4
		94.0	2.1		3.5		0.4
		67.2	1.6		25.5		2.3	3.4
		59.5	2.2		34.4		0.8	3.1
Y16a	Yacheng Formtion	80.2	1.4		13.7		1.8	2.9
		77.2	1.4		12.3		1.1	8.0
		81.5	2.6		12.7		0.8	2.4
		77.4	1.7		16.8		1.2	3.1
		58.4	1.4		26.4	1.9	3.7	8.2
		66.7	2.0		20.8		3.8	6.7
		86.0	2.5		7.7		1.9	1.9
		71.9	1.7		21.5		1.2	3.7

Note: Testing standard: coal maceral, classification of macerals for bituminous coal (GB/T15588-2013).

Within the core of the Yacheng Formation, from Well Y15a and Well Y16a in the Yacheng Ridge area, a coal seam floor of fine sandstone can be observed (Figure 2), which indicates that the development of the coal seams was not genetically related to the underlying strata (Li et al., 2010). It has been speculated that the aforementioned coal seams are in fact allochthonous coal formed by storm deposition (Hu et al., 1998; Kreisa and Bamback, 1982). In the study area, the current research regarding the distribution of the (micro) allochthonous coal remains relatively weak. The previous research experiences have analyzed the hydrodynamics, topography, and other conditions which are known to be favorable for the development of allochthonous coal. It has been speculated that allochthonous coal deposits are mainly distributed in (fan) delta-tidal flat-offshore facies before the peat swarm of the chenier plains and are formed due to the transportation of the peat from the peat swamps via waves or storms and then deposited and stacked in subtidal low energy belts below the average tidal baseline or shallow (estuarine) low energy environments below the normal wave base. However, in sand flats, subtidal high-energy belts, foreshore areas, and onshore high-energy belt facies, the peat was not able to accumulate due to strong hydrodynamic activities. Therefore, it is believed that allochthonous coal would not likely have developed in those particular areas (Hu et al., 1998).

In the current study area, in addition to the coal seams, dark mudstone and carbonaceous mudstone were also found to have developed in the swamps of (fan) delta plains, tidal flats, coastal plains, plains behind the chenier, and limited lakes. These mudstone deposits were observed to be located between the coal seams in a vertical sedimentary sequence. It was found that on the plains, their distribution ranges were relatively consistent with the coal seams. Also, the organic matter was found to mainly consist of terrigenous higher plants (Table 1). Therefore, these deposits were not separately discussed in this research study.

Terrigenous marine mudstone

The terrestrial marine mudstone was found to be another important source rock of the Oligocene in the Qiongdongnan Basin. The terrigenous marine mudstone which is deposited in neritic environments is another major type of source rock. It is known to have organic matter mainly sourced from terrigenous higher plants (over 50%) and little from marine-derived organisms (Li et al., 2011a, 2011b; Liu, 2010). It has been observed that the organic matter of terrestrial higher plants tends to be distributed in a dispersed state. The terrestrial marine mudstone deposits are widely distributed within oceanic areas, with large thicknesses and mass, and the organic matter abundance is generally medium-high (Cai et al., 2003; Han et al., 2016). These types are considered to be medium-good source rock and are conducive to the generation of natural gas (Table 3).

Table 3.

Maceral composition of organic matter and contents of various organic matters in marine mudstone from Yacheng Formation in the typical wells of Qiongdongnan Basin.

Typical well	Typical sedimentary facies	TOC (%)		Sapropelite (%)		Vitrinite + inertinite + liptinite (%)		Marine TOC (%)	Terrigenous TOC (%)
Typical well	Typical sedimentary facies	(Min–Max)/sample number	AVG	(Min–Max)/sample number	AVG	(Min–Max)/sample number	AVG	Marine TOC (%)	Terrigenous TOC (%)
S35	Front fan delta/tidal flat	(0.54–1.22)/18	0.83	(9.4–56.9)/15	32	(43.2–90.7)/15	68	0.27	0.56
L21	Tidal flat/Lagoon	(0.13–2.46)/60	0.50	(10.2–60.7)/31	31.4	(39.3–89.8)/31	68.6	0.34	0.16
C28	Shallow sea	(0.49–1.05)/22	0.84	(27.5–57)/9	39	(41–72.5)/9	61	0.33	0.51
L4	Shallow sea	(1.24–1.46)/2	1.35	(28.3–53.7)/2	41	(46.3–71.7)/2	59	0.24	1.08
S7	Littoral facies	(1.32)/1	1.32	(14.3–23.6)/3	18	(76.4–85.7)3	82	0.24	1.08
Y11	Front fan delta/Tidal flat	(0.04–0.89)/47	0.52	(0–19.2)/3	6.4	(81.8–100)/3	93.6	0.03	0.49

Note: The proportion of sapropelite + inertinite + liptinite in total maceral composition generally represents the proportion of terrigenous and marine organic matter in TOC.

Testing standard: coal maceral, Classification of macerals for bituminous coal (GB/T15588-2013).

Within the source rock of the Oligocene Yancheng Formation in the Qiongdongnan Basin, the coal seams are mainly distributed in the swampy environments of the (fan) delta plains; upper and intertidal zones of the tidal flats and the coastal plain areas (Li et al., 2010). The dispersed organic matter in the terrestrial marine mudstone has been found to have mainly been sourced from river mouth water input. It is known that after the organic matter enters the sea, it is re-transported and transformed by a variety of water bodies, such as storm back-flow, gravity flow, coastal flow, and so on (Correggiari et al., 2001; Schmidt et al., 2010) as well as the influences of topographical slopes and other factors. These areas are known to have wide distribution ranges and large thicknesses (Cai et al., 2003; Correggiari et al., 2001; Li et al., 2018; Peters et al., 2000; Tesi et al., 2008).

It has previously been determined that the distribution ranges of terrestrial marine mudstone have a good negative correlation index with topographical slopes, and the correlations are generally very good, such as the Miocene deposits in the Mahakam Delta of Kalimantan in Southern Borneo (Peters et al., 2000) and the modern deposits in the Po River Delta of Italy (Tesi et al., 2008). This has been found to be very helpful for the quantitative predictions of the distribution ranges of terrestrial marine mudstone. However, the processes involved in transporting and depositing terrestrial dispersed organic matter in water are very complicated, and there are known to be many influencing factors. At this time, the sedimentary characteristics and distribution ranges of terrestrial marine mudstone still require further examination.

Fault depression structural controls of the three toruliform coal-measure source rock belts

In the current study, in accordance with the analysis results of nearly 40 drilling wells, along with logging data and the inversion analysis results of the wave impedance of several seismic sections in the study area (Li et al., 2011d), the coal measures in the Qiongdongnan Basin were determined to be mainly developed in the (fan) delta environments of the lower Oligocene Yacheng Formation (for example, the core coal in Well Y15a and Well Y16a of the Yacheng Ridge area). Second, the tidal flat environments (such as those of Well Y11 in the Yaibei Sag and Well L21 in the Beijiao Sag) and the coastal plain areas were observed to have certain coal-forming effects (as indicated by Well S35). Among those environments, the fault bends, fault groove positions, and fault endings at the edges of the sags were found to be the most favorable areas of the developed coal-bearing (fan) delta environments (Li et al., 2010, 2018).

It was observed that under the control of four large fault belts (Nos. 5, 2, 13, and 15), three coal-measure source rock zones in a toruliform distribution had been formed in the transitional zones from the uplifts or structural heights to the sag of the basin. These included the coal-measure source rock belts in the northern margin of northern fault depression, as well as those in the northern margin of the central fault depression, and in the southern margin of the central fault depression, as detailed in Figure 5. Some of the aforementioned coal-measure source rock belts had directly revealed the development of coal-measures through drilling cores or cuttings (Li et al., 2010). Also, the identification of some developed coal-measures was based on comprehensive logging methods which were then verified by coal-bearing drilling processes (Li et al., 2011d). In other cases, the existence of coal-measures had been successfully achieved in accordance with the results of comprehensive seismic inversion methods, which had been subsequently verified by coal-bearing drilling processes (Li et al., 2011d). In summary, it has been confirmed that the aforementioned three zones were the most favorable for the development of coal-measure source rock in the study area.

Figure 5.

Three coal-measure source rock belts in Yacheng Formation of Qiongdongnan Basin.

Figure 6.

Thin coal seams in the Yacheng Formation revealed by Well Y16a in Ya’nan Sag.

Coal-measure source rock belts in the northern margin of the northern fault depression

The coal-measure source rock belts in the study area were further divided into eastern and western sections. The eastern section was distributed in the north slope of the Songdong Sag, which had the characteristics of a “northern fault and southern overlap.” Meanwhile, the western section was mainly distributed in the northern slopes of the Yabei and Songxi Sags, which also displayed the characteristics of a “northern fault and southern overlap” (Li et al., 2010), as detailed in Figure 5.

During the deposition of the Yancheng Formation, the No. 5 boundary fault in the northern section of the Yabei Sag had shown activities with major intensities, and a small-scaled fan delta had developed in the roots of the descending disk. The deep depression was dominated by a lagoon-type deposition, and the southern gentle slope was dominated by a tidal flat deposition. The grayish-black mudstone of the Yancheng Formation had developed as a horizontal bedding, which was rich in plant stems and leaf fossils, mangrove palynology, and so on (Zhang et al., 2017). A method of comprehensive identification of coal seam by logging curves was applied, and 35 coal seams were identified in Well Y11 of Yacheng Formation, with a total thickness of 16.8 m (Li et al., 2011d).

The results of the drilling (S34) discovery and seismic analysis of the Songxi Sag showed that the sag had been filled with coarse clastic rock during the sedimentary period of the Yacheng Formation. This rock mainly consisted of fine conglomerate, peddled coarse sandstone, coarse sandstone, and coarse clastic fan delta deposits. The sedimentary center of the sag had inclined to a southern gently sloped belt area and had developed into lagoon-tidal flat sedimentation, which was determined to be a favorable location for coal formation (Li et al., 2010).

During the sedimentary stage of the Yancheng Formation in the Songdong Sag, a narrow-banded fan delta group had developed along the southern No. 6 fault zone. Also, the main tidal flat deposits had developed in the northern gentle slope belt, and a large delta had developed in the northeastern gentle slope belt. These areas were also considered to be favorable locations for coal formation (Li et al., 2010).

The zones of the (fan) deltas and tidal flats were found to be well developed in the study area, which are known to be beneficial to the wide development of coal seams. The lagoon areas were small in scale, and the terrestrial marine mudstone which extended toward the lagoon was observed to be well developed, which was considered to be beneficial to the development of coal-measure source rock masses.

Coal-measure source rock belts in the northern margin of the central fault depression

The coal-measure source rock belts of the study area were mainly distributed in the Ya’nan Sag and in the north margins of the Lingshui, Songnan, Baodao, and Changchang Sags (Li et al., 2010), as detailed in Figure 4.

It has been determined that during the sedimentary period of the Yacheng Formation in the Early Oligocene period, the No. 3 fault in the northern section had been active, and a slope trench landform which was high in the north and low in the south had been formed. In the Yacheng Ridge area on its western side, the provenance of Hainan Island had entered the Ya’nan Sag from the northwest and had subsequently formed the large-scale Yacheng Ridge Delta (Figure 4). In recent years, there have been many drilling activities conducted (for example, the Y15, Y16, Y17, Y20, Y21, Y15a, and Y16a Wells) in the Yacheng Formation for the purpose of exposing coal seams (Li et al., 2010). It was found that more than a dozen coal seams (Figure 6) had been revealed in the cores of the third section of the Y15a and 16a Wells. The coal seams were found to have the characteristics of thin thicknesses (several centimeters to dozens of centimeters), many layers, and poor stability. The coal was observed to be semi-bright and semi-dark types, with high densities and high content levels of inorganic components (Li et al., 2010; Zhang et al., 2010).

In addition, fan deltas were found to have developed in the eastern section of the No. 3 fault in the northern margin of the Ya’nan Sag; onshore and tidal flat facies had developed in the Lingshui low uplift areas in the northern margins of the Lingshui Sag; offshore deposits were observed in the Songtao uplift areas, which were a transitional zone from the Lingshui Sag to the Songnan Sag; fan deltas existed in the downthrown walls of the No. 2 Fault in the northern part of the Songnan Sag; and some deposits were observed in the fan deltas and offshore facies in the northern sections of the Baodao and Changchang Sags. These sedimentary facies were believed to be favorable for the formation of coal. According to the wave impedance inversion analysis of the seismic data of coal seams which had been verified by coal-bearing drilling well activities, it was also believed that coal seams were likely to have been developed in the aforementioned areas (Li et al., 2010).

At the present time, only a few wells have been drilled in the coal-measure source rock belts of the study area. However, Well C28 in the northern margin of the Changchang Sag is known to have encountered coal seams in the Yacheng Formation.

Due to the lack of drilled wells in the Yacheng Formation in this coal-measure source rock belts, only Well C28 in the north of Changchang Sag is effectively the first section of Yacheng Formation which provides information regarding potential coal seams in the area. This well mainly contains gray- gray–black mudstone (TOC generally greater than 0.6%), and the organic matter is known to be mainly sourced from terrestrial higher plants. Therefore, the area is considered to belong to the category of terrestrial marine mudstone, as detailed in Table 3. Coal-formed gas was previously found in the Baodao 19–2 tectonic block of the Baodao and Songdong Sags. The source rock of the gas was observed to be the underlying Oligocene coal-measure source rock (Han et al., 2013), which also had reflected the development of coal-measure source rock near the fan delta along the No. 2 and 3 faults of the study area.

Coal-measure source rock belts in the southern margin of the central fault depression

The coal-measure source rock belts in the study are found to be mainly distributed in the southern margin of the Lingshui-Ledong Sag; the periphery of the Beijiao Sag; and the southern margin of the Baodao-Changchang Sag, as shown in Figure 4 (Li et al., 2010).

It has been determined that during the sedimentary stage of the Early Oligocene in the Yacheng Formation, a large fan delta had developed in the Ledong Sag, which was controlled by the southern No. 13 fault (Wang et al., 2014b). In the southern region of the Lingshui Sag, the topography was observed to be relatively gentle, and a delta had also developed in that area (Guo et al., 2016). The Beijiao Sag presented a typical southern fault and northern overlap pattern, and the delta areas were mainly developed in the southern section of the sag. Also, fan delta-tidal flats were observed to be mainly developed in the northeastern regions of the sag, and a central lagoon was observed (Zhang et al., 2013). In the areas with relatively gentle topography at the ends of the faults in the study area, large (fan) deltas were found to have developed (Zhang et al., 2012b).

In the Yacheng Formation, only a few of the coal-measure source rock belts have currently been drilled. These include Well L21 on the northern edge of the Beijiao Sag; Well S35 on the southern edge of the Lingshui Sag; and Well L4 on the southern edge of the Baodao Sag.

The second and third sections of the Yacheng Formation include Well L21 of the Beijiao Sag and are considered to be fan delta-tidal flat sedimentation areas. The first section of the Yacheng Formation has been found to gradually transition to lagoon-type sedimentation. The coal seams are known to be mainly located in the lower part of the first section of the Yacheng Formation and upper part of the second section of the Yacheng Formation. At the present time, 10 layers of coal seams have been found during logging activities, for a total of 11.4 m (Zhang et al., 2013). In this study, an integrated logging data identification method (Li et al., 2011d) was used to identify 23 coal seams with a total thickness of 26.8 m. The third section and the lower part of the second section of the Yacheng Formation (Well S35) in the southern section of the Lingshui Sag have been determined to mainly consist of tidal deposits. Also, the upper parts of the first and second sections of the Yacheng Formation are known to gradually transition into inner shallow sea deposits, and the slack coal deposits have been found during logging activities. Furthermore, Well L4 in the southern region of the Baodao Sag had revealed that part of the strata in the first section of Yacheng Formation were coastal plain deposits. However, the coal-bearing properties of the Yacheng Formation remain unclear. It has been determined that the impedance inversion of the seismic data indicated that there was a strong likelihood of coal-measure source rock being present in the area (Li et al., 2011d).

In addition, within the southern part of the Lingshui Sag, the Lingshui 17–2 coal-formed gas field was discovered. The source rock of the gas was determined to be mainly lower Oligocene Yacheng Formation coal-measure source rock, which implied that the development of coal-measures in that area had indeed occurred to some extent (Liang et al., 2015).

Characteristics of the major gas accumulation belts

Organic macerals and their hydrocarbon characteristics

During the process of the thermal evolution of hydrocarbon generation, the contributions to hydrocarbon generation of different types of microstructures are known to vary. The hydrocarbon efficiency ratio of the shell group, mirror group, and inert group has been confirmed as approximately 3:1:0.71 and the hydrocarbon generation capacity ratio as approximately 3.3:1:0.8 (as per the internal data of the Changing Petroleum Development and Design Institute). The coal dominated by the mirror group is known to be conducive to generate gas, and the coal dominated by the shell group (including the sapropelinite) has been confirmed to be conducive to oil production. Meanwhile, the coal dominated by the inertinite will not be conducive to hydrocarbon production, and the difference between the total hydrocarbon yields of the hydrogen-poor coal and the hydrogen-rich coal could potentially be up to five times more (Liu et al., 2000; Nail et al., 2016).

The content levels of the organic components in the coal deposits of the Yacheng Formation in the Yacheng Ridge Area have been determined to be high, with an average content of 67%. Also, the organic maceral of the coal has been observed to mainly be vitrinite, with an average content of more than 90%. This is followed by inertinite, and almost no liptinite has been observed. It is known that this type of coal is conducive to the generation of natural gas (Zhang et al., 2010).

Thermal evolution

The geological times, temperatures, and thermal effects which source rock experience will affect the thermal evolution, maturity, and hydrocarbon generation of the organic matter (Huang et al., 2012; Yang et al., 2003). High geothermal temperatures not only facilitate hydrocarbon generation but also accelerate the cracking of residual hydrocarbons in source rock masses. This results in the cracking of the hydrogen-rich organic matter into natural gas to its full extent, thereby improving the hydrocarbon generation efficiency and hydrocarbon generation capacity of the source rock (Feng et al., 2015; Singh et al., 2016; Sun et al., 2005; Zhang et al., 2010). The kerogen of the Yacheng coal-measure source rock has been determined to be Type II₂-III, and the R^o_max of the source rock may potentially reach 0.6%. It has been found that small amounts of oil and gas were initially produced, and when the R^o_max value reached 1.3%, the gas had begun to massively generate. When the value of the R^o_max reached 3.0%, the peak of the gas production was achieved, and when the value of the R^o_max reached 4.38%, the gas production had reached its limit (Xiong et al., 2014).

The thermal evolution degree of Qiongdongnan Basin is quite different, and the overall thermal evolution degree is relatively high in the sag. The main reason is that the depth of the strata is large, which can exceed 7500 m (e.g. Ledong Sag). The secondary reason is the thermal action of magmatic intrusions, which improves the degree of thermal evolution in local areas (e.g. Changchang Sag). The R^o_max of the source rock of the central fault depression in the Qiongdongnan Basin was generally greater than 2.0%. However, the R^o_max could potentially be more than 4%. The R^o_max of the source rock in the southern margin of the fault depression was mainly distributed between 0.6 and 2.5%. It was found that the hydrogen-rich components in the source rock could potentially produce oil, and the hydrogen-depleted components could produce natural gas. The R^o_max of the source rock in the northern fault depression of the study area was determined to be mainly distributed between 0.6 and 2.1%, which was similar to the southern margin of the fault depression, and both oil and gas had been produced, as detailed in Figure 7 (Wu et al., 2013; Zhang et al., 2010).

Figure 7.

Thermal evolution degree of coal-measure source rocks of Yacheng Formation in Qiongdongnan Basin.

Distribution of the gas accumulation belts

The co-control theory of “source rock and heat” proposes that potential source rock masses are the internal causes of hydrocarbon generation, and thermal actions are the external causes. It has been observed that the two are indispensable and mutually coupled in order to jointly control hydrocarbon generation, scale, phase (oil or gas) states, and regional distribution patterns. That is to say, source rock and heat jointly control hydrocarbon generation (Lv et al., 2014; Singh et al., 2015; Zhang, 2012; Zhang et al., 2014). The Yacheng Formation of the Qiongdongnan Basin has three coal measure source rock belts, which are located in the northern margin of the northern fault depression; the northern margin of the central fault depression; and the southern margin of the central fault depression. In the study area, after the organic matter of the source rock was subjected to a certain thermal evolution, three (potential) gas accumulation belts had been formed under certain reservoir and capping conditions (Figure 8) as follows.

Figure 8.

Three gas accumulation belts in the Qiongdongnan Basin.

Figure 9.

Formation model of Yacheng 13–1 gas field in Qiongdongnan Basin (see Figure 4 for location).

Gas accumulation belt in the northern margin of the central fault depression

The gas accumulation belt in the northern margin of the central fault depression is known to extend from the Ya’nan Sag to the Lingshui low uplift and to the eastern section of the northern margin of Changchang Sag along the No. 2 fault belt. The gas accumulation belt has been found to be buried at different depths, and variations in the maturity of the source rock have been observed. Generally speaking, the source rock in the central and western regions of the gas accumulation belt has reached an over-mature stage, and the maturity levels toward the eastward region have been found to gradually decrease. Furthermore, the eastern part of the Changchang Sag has been found to have the lowest maturity rate and shallow burial levels (Figure 8).

The reservoirs of the gas accumulation belt mainly include the sandstone in the third sections of the Lingshui and Sanya Formations. The regional caprocks mainly include mudstone in the first and second sections of the Linshui Formation and superjacent in the Meishan Formation. The trap styles mainly include draping anticlines, fault noses, and fault blocks as well as composite traps under the tectonic setting (Lv et al., 2011, 2016; Tao et al., 2006). The above-mentioned traps were found to have good combinations of reservoirs and caps, and it is known that the key to natural gas pool formation is whether or not highly mature coal-measure source rock masses are present.

An example of such a formation is the western section of the Yacheng Ridge area in the Ya’nan Sag. This area is located in the uplift area between the Qiongdongnan Basin and the Yinggehai Basin. Therefore, in accordance with the traditional view of “exploring uplifts by determining depressions,” it has been determined that, based on the gas seedlings in the Yinggehai Basin and the seismic reflection highlight anomalies on the Yacheng Ridge, the Yacheng 13–1 drape structure on the Yacheng Ridge was a favorable gas-bearing structure. Subsequently, a gas field with a reserve of nearly 100 billion squares was drilled. The comparative analysis results revealed that within the trap zone of the Yacheng 13–1 gas field, the Oligocene Yacheng Formation had developed delta coal seams and terrestrial marine mudstone deposits. It is known that since the Late Miocene period, these coal measure source rock masses have continued to be heated and now have a high maturity rate, which has resulted in the generation of large amounts of natural gas. Moreover, the reservoir and traps in the aforementioned area have been found to be very well developed and in good configuration. Furthermore, a gas source fault connecting the source rock and the reservoir is also known to exist, which in turn has formed the Yacheng 13–1 gas field (Figure 9).

Gas accumulation belt in the southern margin of the central fault depression

The gas accumulation belt in the southern margin of the central fault depression starts from the southern margin of the Ledong Sag; bypasses the southern margin of the Lingshui Sag and Songnan low uplift (including its pinching-out end); and reaches to the southern tectonic belt of the Changchang Sag (referred to as the Changnan Tectonic Belt). The current drilled wells in the gas accumulation belt are mainly concentrated in the southern margin of the Lingshui Sag. Three large and medium-sized gas fields (Lingshui 17, Lingshui 25, and Lingshui 18) have been discovered. Each of the Lingnan low uplifts, including the Beijiao Sag, Songnan low uplift, and Changnan Structural Belt, now have drilled wells, and it has been found that encounter hydrocarbon source rock or oil and gas layers are often present.

The thermal evolution degrees of source rock are known to vary greatly, and the degree of thermal evolution tends to gradually decrease from the sags to the uplifts. In other words, the centers of the sags tend to be over-mature; the transition zones tend to be highly mature; and the uplift zones are generally characterized as being immature or having low maturity (Figure 7). The maturity of the Yacheng Formation in the central fault depression has been determined to be generally high. The source rock of the Yacheng Formation in the Ledong and Lingshui Sags is known to be over 4.0% in maximum vitrinite reflectance and had roughly entered the peak stage of gas generation in the deposition period of the Yinggehai Formation. The gas testing logs of the drilled wells of the central canyon in the northern part of the Lingnan low uplift showed that the natural gas had very low content levels of heavy hydrocarbons. These results indicated a dry gas, with a C₁/(C₁ – C₅) ranging between 0.97 and 0.98. However, it is different from the biogas generated in the overlying strata (the drying coefficient is greater than 0.99), which can preliminarily exclude the possibility of biogas. According to the MDT test analysis of six gas samples of LS-A gas reservoir (Table 4), the CH₄ content is 91.15–92.87%, containing a certain amount of heavy hydrocarbon, C₂₊ is 6.07–8.02%, and C₁/C_1–5 is 0.92–0.94. Also, from the analysis of carbon isotope of the gas samples of LS-A gas reservoir, it is found that the δ¹³C₁ is between –39.4 and –38.8‰ (Figure 11), similar to that in the Yacheng 13–1 gas field. Gas composition and carbon isotope composition of gas reservoir suggest that the gas is high-mature coal-formed gas derived from the deep and mature coal measure source rock of Yacheng Formation (Wang et al., 2015).

Table 4.

Gas composition and carbon isotope composition of LS-A gas reservoir (Wang et al., 2015).

Stara	Depth (m)	Component content (%)								Drying coefficient (%)	δ¹³C (‰)
Stara	Depth (m)	N₂	CO₂	C₁	C₂	C₃	C₄	C₅	C₆₊	C₁/C₁_–5	C₁	C₂	C₂	C₄	CO₂
Huangliu formation	3339	0.558	0.308	91.116	5.036	1.486	0.681	0.364	0.55	92.332	–39.4	–26.2	–24.3	–	0.3
	3339	0.554	0.305	91.155	5.039	1.487	0.685	0.273	0.501	92.413	–39.2	–26.2	–23.8	–	–7.4
	3352.5	0.565	0.318	91.367	4.941	1.437	0.655	0.248	0.469	92.619	–38.8	–26.0	–23.7	–	–8.5
	3391	0.552	0.318	91.533	4.954	1.386	0.624	0.238	0.395	92.706	–39.2	–26.0	–24.1	–	–9.6
	3391	0.537	0.304	91.702	4.951	1.362	0.610	0.235	0.299	92.759	–39.0	–25.9	–23.8	–	–9.2
	3407	0.308	0.757	92.868	4.323	0.998	0.406	0.148	0.192	94.050	–	–	–	–	–

Note: Testing standard: gas chromatography, analysis of natural gas by gas chromatography (GB/T13610-2003); stable carbon isotopes of organic matter, organic geochemical measurements geological samples—method for the analysis the organic matters for stable carbon isotopic composition (GB/T 18340.2–2001).

In the study area, there were four sets of reservoirs in the gas accumulation belts as follows: (1) a turbidite sand-body in the central canyon of the Upper Miocene Huangliu Formation; (2) a turbidite fan sand-body in the Miocene Sanya Formation; (3) a turbidity water channel sand-body and a low fan sand-body in the Lingshui Formation; and (4) an organic reef. The regional caprock included the hugely thick Pliocene-Quaternary marine mudstone and regional mudstone in the middle of the Miocene Meishan Formation, along with other local caprock formations. The oil and gas migration channels were the diapir zones or fault zones caused by the hydrocarbon overpressure deep within the Yacheng Formation as well as the sand deposits and unconformity characteristics of the transport layers.

The turbidite channel sand in the central canyon was determined to be composed of multiple independent sand bodies, and the pods were observed to be intermittently distributed, mainly in the lithologic traps. On both sides of the turbidite sand reservoir, the well-developed reservoir-cover combination was found to be conducive to the development of hydrocarbon accumulations, which had been confirmed by several oil and gas fields (Tao et al., 2006; Zhang et al., 2018). For example, the Lingshui 17–2 gas field activities had discovered that in the Central District of the Central Gorge, a gas reservoir composed of several adjacent and isolated gas reservoirs exists. It has been proposed that the key to the effective reservoir formation in the Lingshui 17–2 trap was the development of a coal-series (fan) delta system in the lower Yacheng Formation. In that area, the coal and terrigenous marine mudstone deposits were very developed; the hydrocarbon source rock masses were abundant; and the thermal evolution of the organic matter was high, with a large number of gas generation conditions (Tao et al., 2006; Wang et al., 2016; Zhang et al., 2016), as detailed in Figure 10.

Figure 10.

Formation model of Lingshui 17–2 gas field in Qiongdongnan Basin(see Figure 4 for location).

Figure 11.

Carbon isotope analysis determines the type of natural gas in different areas (Wang et al., 2015).

Furthermore, there are known to be many traps in the gas accumulation belt of the southern margin of the central fault depression, and the exploration degree remains relatively low. It has been speculated that the favorable traps are located in the areas where the coal-measure source rock (coal seam and terrestrial marine mudstone) has been observed to have developed, and the thermal evolution degrees are relatively high. Therefore, those areas are considered to be prospective areas for future coal-formed gas explorations.

Potential gas accumulation belt in the northern margin of the northern fault depression

The gas accumulation belt in the northern margin of the study area is known to be distributed along the No. 5 fault zone of the basin. It has been found that fan deltas have often developed along the No. 5 fault, and the larger deltas have been determined to only occur in the northern slopes of the Songdong Sag. The source rock in the area has been observed to mainly include the coal-measure source rock of the (fan) delta-tidal flats. The coal-measure source rock belt of the fan delta-tidal flats in the northern margin of the northern fault depression of the Qiongdongnan Basin has a low thermal evolution degree, and the majority of the R^o_max is less than 1.3%. Therefore, it has not entered the mass hydrocarbon generation stage and is not conducive to the formation of coal-formed gas reservoirs (Figure 7). The terrigenous marine mudstone belt in front of the coal-bearing (fan) delta has a large burial depth and a relatively high maturity. Therefore, it may be possible that some coal-formed gas has been generated. However, it is believed that the gas reserves will generally be small. The aforementioned understanding of the situation in the study area has been confirmed by drilling activities, and several coal-formed gas reserves were previously encountered within multiple structures of the study area, though on a smaller scale.

The northern fault depression has a large area (up to 3350 km²) and good source rock conditions. Six wells have been successively drilled in six structures as follows: the drape structure (Yacheng 14–1, Yacheng 7–4, Lingshui 2–1); rolling anticline (Yacheng 8–2); and large fault nose structure (Yacheng 8–1, Songtao 31–2). However, apart from the discoveries of a 1.4-m thick oil layer in Well Y17 and a 2.5-m thick gas layer in Well Y12 in the superjacent Sanya Formation, no oil and gas layers have been located in the other wells. The results of the post-drilling analysis revealed that, although the source rock was relatively developed in the area, it was low in maturity. The source rock had not yet entered a stage of substantial hydrocarbon generation and was not equipped with the thermal evolution conditions for hydrocarbon generation.

Therefore, although this potential gas accumulation belt did contain both source rock and traps, as well as good combinations of reservoirs and caprock, it was limited in exploration potential due to its low thermal maturity levels.

Conclusions

The lower Oligocene Yacheng Formation coal-measure source rock (coal seams and terrigenous marine mudstone deposits) is the most important source rocks in the Qiongdongnan Basin. The basin prototype is known to have the characteristics of a typical marine-continental transitional facies fault basin.

The Qiongdongnan Basin had extended strongly during the Early Oligocene period, during which time the northern fault depression and the central fault depression were formed. It can be seen that in the northern slope belt of the northern fault depression, and also in the southern and northern slope belts of the central fault depression, three macroscopic coal-measure source rock belts have formed in the (fan) deltas, tidal flats, plains behind the chenier, and coastal plains, with a certain range extending into the sea.

Following the sedimentation which occurred during the Lower Oligocene period in the coal-measure source rock of the Qiongdongnan Basin, the basin had continued to sink and be heated, and the thermal evolution of organic matter had gradually increased. The coal-measure source rock on the northern and southern sides of the central fault depression had experienced a high thermal evolution degree and had entered a phase of mass gas production. Also, gas accumulation had occurred in the overlying strata through gas channels, and two large gas accumulation belts had been formed. Therefore, it is believed that the exploration potential in the aforementioned area is extremely large, and the trap zones near the mature coal-measure source rock are favorable for future explorations. The coal-measure source rock in the northern margin of the northern fault depression has been determined to have experienced only weak heating conditions, and the majority of the source rock remains immature at present. It has been found that immaturity of the source rock is not conducive to the generation of large amounts of gas. Therefore, the exploration potential for natural gas in the aforementioned area is believed to be limited.

Footnotes

Acknowledgements

Careful reviews and constructive suggestions of the manuscript by anonymous reviewers are also greatly appreciated.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work is supported by National Science and Technology Major Project “Potential of oil and gas resources in deep waters of South China Sea and exploration direction of large- and medium-sized oil and gas fields” (2016ZX05026007-004); National Natural Science Foundation of China “Comparative study on genetic mechanism and evolution model of lamina in fine sediments under backgrounds of different basins” (41672096); “The Oligocene in the southeastern qiongdongnan basin also wanted to take advantage of the mechanism of organic matter accumulation in the assemblage” (41872172); Research and innovation team support plan of Shandong University of Science and Technology (2018TDJH101); and the open fund project of Key Laboratory of Resource Exploration Research of Hebei Province.

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