Abstract
Deep Basin Gas is short for deep basin gas accumulation. It is an abnormal gas accumulation whose formation conditions, trapping mechanism and distribution are different from those of normal gas accumulations. Deep basin gas accumulation is characterized by gentle dip angles, subnormal pressure, gas-water inversion and co-occurrence of reservoir and source rock. The fundamental conditions favourable to the formation of deep basin gas accumulation include a plentiful gas source, tight reservoir and tight seal under the reservoir.
Two balances are the prerequisite for formation and preservation of deep basin gas accumulation. One is the force balance that occurs between the upward forces, including gas volume expansion pressure and buoyancy, and the downward forces including hydrostatic pressure and capillary pressure. The other is material balance that occurs between the supply amount of gas and the escaping gas. If the amount of gas charging the reservoir is more than that of escaping gas, the distribution range of the accumulation will expand up to the boundary limited by the force balance; and vice versa, a lower supply will cause shrinkage of the range.
The force balance determines the theoretical maximum range of deep basin gas accumulation. In this range, gas expelled from the source rock can be accumulated to form a deep basin gas pool. The greater the amount of gas that is expelled from the source rock, the larger will be the distribution range of deep basin gas accumulation. Beyond this range, gas that is expelled from the source rock has no choice but to migrate under the force of buoyancy to form a normal gas accumulation.
The equation of force balance predicting the theoretical maximum range of deep basin gas is
where, Sw ρw, g, ρg represent saturation of water in reservoir, density of water, acceleration of gravity and density of gas, respectively; T and R are, respectively, subsurface temperature of gas and gas constant; L represents the lateral distance from the depth of boundary force balance to the maximum depth of the depression centre.
When the thickness of the reservoir(Hs), grain size of sandstone(D), porosity(Ø), and dip of strata(α) increase and maximum burial depth of reservoir(Zm) decreases, the likely distribution range of deep basin gas will shrink.
In this paper, based on the mechanism of material balance, the equation calculating the distribution range of a deep basin gas pool in actual geological settings is
It is shown that, in actual geological settings, the distribution range of deep basin gas accumulation will expand with better source conditions (or with an increase of thickness of source rock (Hn), abundance of organic matter(C%), kerogen type(KTI) and thermal evolution degree (Ro)) and with increase of burial rate (SR), burial depth(Zm) and salinity of formation water, but will shrink with increases of age of the reservoir(t), temperature(T), porosity(Ø), permeability (K) and dip of strata().
In the Xiaocaohu region and Well Taican 2 region of the Taibei sag in the Turpan-Hami Basin, stable structural settings, well-developed gas source, tight reservoir and feasible cover are favourable to form a deep basin gas reservoir. Drilling shows that there exist deep basin gas accumulations in Well Taican 2 region and Xiaocaohu sub-sag. For the gas layers drilled in the Jurassic Badaowan formation (J1b) and Xishanyao formation (J2x), there is subnormal pressure generally. The reservoirs outside the gas-bearing range in Hongtai and Gedatai gas field are tight and are interpreted as gas layers by logging and produce gas without water, and so belong to one part of a deep basin gas reservoir. Specially processed seismic data shows that there exists a large amount of natural gas in J1b and J2x of Well Taican 2 region. From the principles of force balance and material balance it is predicted in this paper that the distribution ranges of deep basin gas reservoir in J1b and J2x of Xiaocaohu sub-sag are 600km2 and 750km2, respectively, and the total reserves of natural gas should reach 11.3×1011m3.
The Turpan-Hami Basin, located in northwest China, is of Mesozoic and Cenozoic age, and is a continental coal-bearing intermountain basin. Recently significant amounts of oil accumulations have been found in the Jurassic layer, and are thought be coal-derived oils, generated chiefly from coal-bearing layers of the Jurassic Badaowan formation (J1b) and Xishanyao formation (J2x) (Wu, 1996; 1997; Cheng, 1994; Huang, etc., 1995). Coal is a typical humic organic matter and, although it contains macerals of exinite etc. that generate oil, it mainly generates gas. The large oil accumulation that is found in the basin (in which the source rocks generate gas primarily) indicates that natural gas exploration has a wide realm and high prospectivity. We have studied the formation mechanisms of the natural gas reservoirs that have been found. The pressure and attitude features of the formation are different from those of a normal gas reservoir, and are the same as those of deep basin gas reservoirs reported by others (McMaster, 1983; Gant, 1983; Masters, 1993; Welte, et al, 1984; Gies, 1988; Masters, 1988; Yuan and Xu, etc, 1996; Rong, 1993; Li1 et al., 1997; Jin et al, 19982, Jin and Zhang 1999; Chen, 1998; Dai, 1983; Min et al., 1996; 1998). The conditions of accumulation are analyzed and evaluated with deep basin gas accumulation theory, based on which the potential distribution range of deep basin gas is predicted.