Formation mechanisms,potentials and exploration practices of large lithologic gas reservoirs in and around an intracratonic rift:Taking the Sinian-Cambrian of Sichuan Basin as an example

By examining structures,sediments,reservoirs and accumulation assemblages in the Deyang-Anyue rift and its surrounding area,four new understandings are obtained.First,during the initiation period of Deyang-Anyue rift,multiple groups of faults developed in the rift due to the effect of tensile force,bringing about multiple mound and shoal belts controlled by horsts in the second member of the Sinian Dengying Formation;in the development stage of the rift,the boundary faults of the rift controlled the development of mound and shoal belts at the platform margin in the fourth member of Dengying Formation;during the shrinkage period of the rift,platform margin grain shoals of the Cambrian Canglangpu Formation developed in the rift margin.Second,four sets of large-scale mound and shoal reservoirs in the second member of Dengying Formation,the fourth member of Dengying Formation,Canglangpu Formation and Longwangmiao Formation overlap with several sets of source rocks such as Qiongzhusi Formation source rocks and Dengying Formation argillaceous limestone or dolomite developed inside and outside the rift,forming good source-reservoir-cap rock combinations;the sealing of tight rock layers in the lateral and updip direction results in the formation model of large lithologic gas reservoirs of oil pool before gas,continuous charging and independent preservation of each gas reservoir.Third,six favorable exploration zones of large-scale lithologic gas reservoirs have been sorted out through comprehensive evaluation,namely,mound and shoal complex controlled by horsts in the northern part of the rift in the second member of Dengying Formation,isolated karst mound and shoal complex of the fourth member of Dengying Formation in the south of the rift,the superimposed area of multi-stage platform margin mounds and shoals of the second and fourth members of Dengying Formation and Canglangpu Formation in the north slope area,the platform margin mounds and shoals of the second and fourth members of Dengying Formation in the west side of the rift,the platform margin mound and shoal bodies of the fourth member of Dengying Formation in the south slope area,etc.Fourth,Well Pengtan-1 drilled on the mound and shoal complex controlled by horsts of the second member of Dengying Formation in the rift and Well Jiaotan-1 drilled on the platform margin mound and shoal complex of the North Slope have obtained high-yield gas flows in multiple target layers,marking the discovery of a new gas province with reserves of(2-3)×10^(12) m^(3).This has proved the huge exploration potential of large lithologic gas reservoir group related to intracratonic rift.

Formation conditions of Sinian–Cambrian large lithologic gas reservoirs in the north slope area of central Sichuan Basin,SW China

Based on analyses of characteristics,hydrocarbon charging history and geological conditions for the formation of Sinian-Cambrian reservoirs in the north slope area of central Sichuan paleo-uplift,the natural gas origin,accumulation evolution,accumulation pattern and formation conditions of large lithologic gas reservoirs have been investigated.Through comprehensive analyses of natural gas composition,carbon and hydrogen isotopic compositions,fluid inclusions,reservoir bitumen,and geological conditions such as lithofacies paleogeography and beach body characterization,it is concluded that:(1)The natural gas in the Sinian-Cambrian of the north slope area is mainly oil cracking gas,and different contribution ratios of multiple sets of source rocks lead to different geochemical characteristics of natural gas in different reservoirs.(2)Although the both Sinian and Cambrian gas reservoirs in this area are lithologic gas reservoirs under monocline background,the former has normal-pressure and the latter has high-pressure.There are three types of source-reservoir-caprock combinations:single source with lower generation and upper reservoir,double sources with lower generation and upper reservoir or with side source and lateral reservoir,double sources with lower generation and upper reservoir or with upper generation and lower reservoir.The Permian-Triassic is the main generation period of oil,Early-Middle Jurassic is the main generation period of oil cracking gas and wet gas,and Late Jurassic-Cretaceous is the main generation period of dry gas.(3)The Sinian-Cambrian system of the north slope area has two favorable conditions for formation of large lithologic gas reservoirs,one is that the large scale beach facies reservoirs are located in the range of ancient oil reservoirs or near the source rocks,which is conducive to the"in-situ"large-scale accumulation of cracked gas in the paleo-oil reservoirs,the other is that the large scale mound-beach complex reservoirs and sealing layers of inter beach tight zones match effectively to form large lithologic traps under the slope background.The research results confirm that the north slope area has large multi-layer lithologic gas reservoirs with more than one trillion cubic meters of natural gas resources and great exploration potential.

The Newly Discovered Yanchang Gas Field in the Ordos Basin, Central China

The Yanchang gas field is located in the Ordos Basin of central China to the southeast of the Yishan Slope, covers an exploration area of 2.6× 10~4 km^2 and has approximately 3.5× 10^（11） m^3 of gas reserves. The gas field is dominated by lithologic gas reservoirs but also has a few structural gas reservoirs. Sand bodies were deposited in the Carboniferous Benxi Formation around offshore barrier islands and in distributary channel fillings in the delta front of the P_1s_2 and P_1s_1 divisions of the Permian Shanxi Formation. The P_2h_8 division of the Shihezi Formation contains the main reservoirs. The depths of the reservoirs are between 1 970 and 3 500 m. The Yanchang gas field can be classified as a typical tight sandstone gas reservoir filed because its porosity is lower than 10% and permeability lower than 1 mD. The discovery and development of the Yanchang gas field has led to a great increase in total natural gas reserves in the Ordos Basin. Its exploration has improved methods of locating large gas fields in deep-water depositional environments in the south part of the basin.