Gas Accumulation Mechanism In Deep Carbonate Rocks Of Reef-Beach Facies

Deep oil and gas reservoirs of reef-beach facies feature high maturity of organic matter and strong heterogeneity of reservoir rocks;this makes it difficult to perform hydrocarbon and source rock correlation and investigate complicated fluid activities in different periods and the differences in deliverability and gas-water distribution.Our efforts focus on the recovery of gas accumulation in deep reservoirs of reef-beach facies in the Changxing Formation in southeastern Sichuan and gas accumulation controls.We accomplish comprehensive assessment of potential Permian source rocks in southeastern Sichuan using source rock pyrolysis,organic carbon analysis,and organic carbon element analysis and present geochemical indicators and methods of parameter selection to address the issues of heterogeneity and high maturity.In view of most routine geochemical indicators which are useless in the case of deep oil and gas reservoirs of reef-beach facies with high maturity and heterogeneity,we adopt several methods for hydrocarbon and source rock correlation to investigate the genesis and origin of highly mature gas and bitumen in Changxing reservoirs of reef-beach facies in southeastern Sichuan,and we find that such bitumen and gas mainly originated from Permian source rocks.Based on the study of thin sections,fluid isotopes,and inclusions,we investigate the periods and sequence of fluid accumulation in Changxing reservoirs in major structures.Subsequently,we restore the process of hydrocarbon accumulation in Changxing reservoirs of reef-beach facies in southeastern Sichuan,examine accumulation controls,and predict predominant prospects with Changxing gas reservoirs in southeastern Sichuan.The results are detailed as follows.(1)Changxing source rocks in southeastern Sichuan cannot be effective source rocks in view of low TOC content generally below 0.5%.Source rocks in the Wujiaping Formation may be the most effective Permian source rocks in this region due to sapropelic organic matter,high TOC content,and post mature organic matter;the hydrocarbon-generating intensity is high in the north and south and low in the center.The quantity of hydrocarbon generation is 44.29×10¹² m³ and the size of resources is0.393×10¹² m³.Organic matter abundance is low in the source rocks in the Lower Permian Qixia and Maokou Formations.For Lower Permian source rocks,the hydrocarbon-generating intensity is high in the east and west and low in the center;the quantity of hydrocarbon generation is 54.38×10¹² m³ and the size of resources is0.163×10¹² m³.(2)Changxing gas mainly originated from thermal cracking of fossil oil generated by marine Permian source rocks,and residual bitumen in Changxing fossil oil reservoirs was generated by Permian source rocks.In the northern region with thick Upper Permian source rocks,Wujiaping carbonaceous mudstone is the major source rocks for Changxing bitumen.In the mid-south with thin Upper Permian source rocks,Lower Permian marl is the major contributor.(3)For highly to post mature bitumen,bitumen component concentration and saturated hydrocarbon biomarkers may be invalid for correlation due to the influence of thermal maturation and reservoir heterogeneity.In contrast,the parameter pair of aromatic hydrocarbon components with similar elution time and different origins and trace elements may be taken as effective indicators of oil and source rock correlation to address the issues of high maturity and strong heterogeneity.(4)Changxing reservoir rocks mainly consist of limestone,dolostone,and transition rocks including bioclastic limestone(dolostone),calcarenite and dolarenite,crystalline grain dolostone,karst breccia,and bioherm limestone.As per drill cores,bioclastic limestone and bioherm limestone are the major reservoir rock types.Pore space in Changxing reservoirs is composed of intergranular pores,intragranular pores,moldic pores,intercrystalline pores,and geopetal pores;intercrystalline pores are the major pore type.Laterally,predominant reservoirs are related to sedimentary facies.Organic reefs show good petrophysical properties,followed by bioclastic beaches.Effective reservoirs seldom occur in open platform facies and slope facies.(5)Fluid accumulation in Changxing reservoirs occurred in four periods,corresponding to the formation of fine crystalline calcite,medium crystalline calcite,coarse crystalline calcite,and crystalline grain calcite,in 250-245 Ma(Early Triassic Epoch),245-177 Ma(Middle Triassic Epoch to Middle Jurassic Epoch),177-165 Ma(Middle Jurassic Epoch),and 165-157 Ma(Middle Jurassic Epoch to Late Jurassic Epoch),respectively as per inclusion thermometry.Hydrocarbon accumulation mainly occurred in the Middle Jurassic Epoch,corresponding to the formation of medium crystalline calcite to crystalline grain calcite.(6)Changxing reservoirs of reef-beach facies experienced three stages,i.e.hydrocarbon accumulation at the early stage,adjustment at the middle stage,and final shaping(with local destruction)at the late stage.Early accumulation was dominated by reservoir type and the size of source rocks;large-scale accumulation tended to occur at the early stage of organic reefs.Hydrocarbon preservation was dependent on tectonic setting;hydrocarbon reservoirs may be well preserved in synclinal zones and damaged in anticlinal zones with steeply dipping structures and abundant fractures.Slope zones are promising prospects in view of their good preservation conditions and relatively structural highs for hydrocarbon accumulation.