The Research On The Difference Of Xujiahe Formation's Conditions Of Gas Formation In Xiaoquan-fenggu Structure Belt

The paper studies the difference of Xujiahe Formation's conditions of gas formation in Xiaoquan-fenggu structure belt. Based on the predecessor's research and combined with the factors like earthquake, drilling, well logging, logging and data processing and analyzing, the article describes the discovered gas reservoir geological elements like gas reservoir types, reservoir petrophysical characteristics, cap rock characteristics, structural feature and fluid properties(capacity, gas water chemical and physical properties, such as temperature, pressure conditions) on the different tectonic units and layers of Xiaoquan-fenggu structure belt. The most beneficial exploration area and direction is point out based on the difference analyze of accumulation conditions and the geological features difference of each tectonic units and layers.(1)Analysis on Organic microscopic group, the components of hydrocarbon source rocks race and the Biomarker compound shows the Kerogen type at Xiaoquan-Fenggu is primarily typeⅡ,TOC is highly contented in Hydrocarbon source rocks. There is big potential to generate hydrocarbon. The hydrocarbon source rocks at Xujiahe is currently at a high matural stage. From the angle of plane, In the aspects of organic thickness, richness and the hydrocarbon-generating rate, Xin chang area is better than Xinghechang and Fenggu area at upper Xujiahe formation, Fenggu area is better than Xinchang and Hexiangchang area at the fourth of upper Xujiahe formation.(2)Generally speaking, the reservoir of Xujiahe group has a feature of low permeability and low porosity which are different obviously in each segment. Fouth segment of Xujiahe formation is deposited from the front of braided fluvial river delta and the delta plain which leads the sediment particle size get thinner from north to south. The second segment is from underwater distributary channel of braided fluvial river delta's front which leads sand body present withdraws from west to east. Both of the two segments are terrestrial clastic rock deposits which contain clastic quartz sandstone, silty sandstone and even coal seam at some area. From the angle of vertical, at The fourth segment of Xujiahe formation, the sub segment is better in T₃X₄³ sand group,the middle of sub segment is relatively better in T3X47 sand group's physical property, the lower of sub segment is best in T₃X₄⁹sand group's physical property, T3X410 takes the second place. At the second segment of Xujiahe formation, the order of the sand group's physical property is T₃X₂⁴, T₃X₂¹,T₃X₂⁵. From the angle of plane, the sand group's physical property of T₃X₄³ is the best in xinchang coMPared to Fenggu and Hexingchang area. The T₃X₄⁷sand group is found existing in Fenggu area as well as Hexingchang in the format of thin sand layer body which is relatively poor in property. The property order of T₃X₄⁹ sand group is Xichang better than Fenggu, Hexingchange Gaozimiao area is relatively poor. In the second of Xujiahe formation, the objective stratum is T₃X₂¹,T₃X₂⁴ sand group at Xinchang-fenggu area. Among them, the property of T₃X₂¹ sand group is best in Xinchang area, better than Gaomiaozi area, hexingchang and Fenggu area are relatively poor. Gaomiaozi area is better than Xinchang area in the property of T₃X₂⁴ sand group.(3)Through the analysis to 29 Wells'acoustic time and pressure statistics of 38 Wells'measured strata, it shows that xujiahe reservoir section's pressure distribute in a wild range with pressure coefficient from 1.44 2.19. The forth segment of Xujiahe formation is mostly in a strong overpressure or nearly strong overpressure pressure characteristics with an average pressure coeffiecient of 1.90. The straras of Fenggu and Xinchang area are in strong overpressure with a pressure coeffiecient of 2.0. Hexingchang and Gaofuzi area is in a smaller pressure with a pressure coeffiecient of 1.58 only. The second segment of Xujiahe formation is mainly in a medium overpressure pressure characteristics with an average pressure coeffiecient of 1.60. There is no obvious change at the plane except a slightly higher pressure at the tops of Xiaoquan and Wulangquan. There are three overpressure combination types in the research area with at least three overpressure warehouses and two obvious pressure layers. There is not direct relationship between the formation of overpressure warehouse and strata while gas reservoir distribution and overpressure warehouse's distribution are in a close relationship.(4)In the research area, gas is reserved in a good condition with mud strata wildly generate which is excellent in sealing as well as multiple sealing ways could be found there. The research based on the micro developmental characteristics of sealing layer shows that the sealing capacity is quite different in different regions and segments, for example, the second segment of xujiahe formation is much better than the forth. From the angle of plane, the cover layer breakthrough pressure increase gradually from west to east at the forth segment of Xujiahe formation, Fenggu area is the best(>8.5MPa), Xiaoquan area is the worst(<8MPa). Howerver, the result is in opposite in the research of the second segent which is Xiaoquan and Xinchang area are the best(>14MPa), Fengku is the worst(<13MPa). Cover layer breakthrough residual stress research suggests that there are several leak sources in the forth segment of Xujiahe formation's cover layer at the northeast of Xinchang's main structure and the southeast of Fenggu area where don't have preservation conditions. The preservation condition is good at the west of Xiaoquan tectonic, the north of Xianchang's main structure, Hexingchang area and the west of Fenggu's main structure. Besides that, there isn't leak source at the second segment of Xujiahe formation which is in a good reservation condition. Research shows that when the reservoir pressure coefficient is more than 1.9, traps is most likely to be non production layer or dry layer, the cover layer can't work effectively at the vertical direction.(5)Through the analysis on 63 paraffin carbon isotopes, it shows that the gas isδ13C2>-29‰which is mainly coal corruption plant gas. In the Xujiahe formation mostly has carbon isotopic series which are not obvious in mix source characteristics. The hydrocarbon source rock types and gas features are clearly different in the reservoiring combination between the upper Xijiahe formation and lower Xujiahe formation. It's mature hydrocarbon source rocks gas at the upper Xujiahe formation but high maturity post gas at the lower Xujiahe formation.(6)Based on the analyze of regional tectonic evolution, the basic tectonic characteristics of research area and the meticulous depict of fracture system, the paper marks out the tectonic units of upper and lower Xujiahe formation of Xiaoquan-fenggu structure belt, analyzes the basic characteristics for formation and evolution, a conclusion is reached that the second segment of Xujiahe formation is formed earlier and the tectonic highs are located at Xinchang and Hexingchang area, there are fractures existing which connect Xiaotangzi and Maantang, providing a transferring channel which is helpful in the gas reservation. The forth segment of Xujiahe formation is formed relatively later with fractures that are throughout the upper strata, leading the gas leak. As a result, the forth segment is worse than the second segment in reservation.Based on the difference analysis of the accumulation conditions, the anatomy of typical gas reservoir, the systematic analysis of form- reserve -cover-trap-transfer conditions, the paper gives beneficial direction for the exploration: the second segment of Xujiahe formation in Xinchang and Hexinchang area are the best exploration area; The second segment of Xujiahe formation in Fenggu area, the lower sub-forth-segment of Xujiahe formation in Hexinchang and Fenggu area, the upper sub-forth-segment of Xujianghe formation in Xinchang, Hexingchang area are favorable exploration area; the upper sub-forth-segment of Xujiahe formation in Xiaoquan, fenggu area are potential areas.