Research On Hydrocarbon Accumulation Regularity Of Lithologic Reservoirs In Hongliuquan Area Of Qaidam Basin

The history of hydrocarbon exploration and development in Hongliuquan of Qaidambasin, a over-mature exploration area in the Southwestern Qaidam Basin, has been nearly60years, so lithologic reservoir has been the significant target in its recently strategicplanning. According to advanced theory and methodology of high resolution sequencestratigraphy and subtle reservoir exploration, integrated with seismic, drilling, logging andanalytical data, this study established an isochronous framework of high resolutionsequence stratigraphy in the lower member of lower Ganchaigou formation inHongliuquan, elaborated the types and distribution of sand bodies, and formulated itssedimentary models. All of these researches concentrated on sedimentary facies andsystems and viewed the level3of sequence and its related system tracts as a basic analysisunit. And then we analyzed the conditions and dominated factors for hydrocarbonaccumulation, clarified the regulations of petroleum distribution and enrichment,established hydrocarbon migration/accumulation model, and pointed out the potential playsfor lithologic reservoir exploration.The lower member of the lower Ganchaigou formation in Hongliuquan, restrictedbetween T4and T5lineups, is sedimentation of level3of sequence. It can be divided intosix mid-term base-level cycles. MSC1～MSC4are LSTs and MSC5～MSC6are LTSTs.The first flood surface of lacustrine lies in the lower part of MSC5. Viewing the level3ofsequence as basic unit, this study established a local frame of high resolution sequencestratigraphy and analyzed the stratigraphy characteristics.The results indicate that the lower member of lower Ganchaigou formation depositedin the braided delta-lacustrine environment. Sandy conglomeratic bodies are common inenvironments of braided rivers, submarine braided rivers, mouth bar/distal bar, submarinebraided interchannel and shallow lacustrine sand bar. Under the multi-controlling oftectonic background, continued rising of base level, sediment source and Alaer branchingrivers, the major trending of sandy conglomeratic bodies is NE. The percentage of sandand conglomerate in rocks decreases from the lower to the upper of the formation. Thedistribution of sand bodies is characterized with stratabound in vertical profile andfacies-controlling in plane for the dual function of sequence frame and sedimentary facies.Hongliuquan area has excellent conditions for forming lithologic reservoirs （1） thelower Ganchaigou formation in Hongshi depression, closed to Hongliuquan, developshydrocarbon source rocks;（2） the reservoir sand bodies of braided rovers and submarinebraided rivers have low porosity and permeability;（3） the thick mudstone of the upper member of lower Ganchaigou formation is ideal cap rock as well as the updip pinch-out ofsandstone and the variations of physical property and lithology create excellent side seal,which indicate very good conditions for forming lithologic traps;（4） the connectivity ofsand bodies in MSC1is pretty high, especially sand bodies around Hongliuquan fault andQigequan fault as well as argillaceous siltstone, siltstone and sandstone with more than0.3m of thickness,2%of porosity and0.0210^-3um²of permeability can form a validhydrocarbon migration network;（5） faults migrated hydrocarbons in the accumulationperiod and sealed hydrocarbons today while unconformity play limited role in hydrocarbonmigration and accumulation. Consequently, there are three types of migration networks inHongliuquan area （1） the first network between Qigequan fault and Hongdi107well area;（2） the second network among Hongliuquan fault and Hongcan2well area, and （3） thecompound migration network in Hong30well area. The preferential spatial relation ofmigration network is the type of sand-fault.Based on detailed characterization of typical Lithologic reservoirs this studysummarized the distribution regulation and controlling factors of hydrocarbonaccumulation in Hongliuquan. The results show that lithologic reservoirs are preferential inthis area. It includes three types: updip pinch-out of sandstone reservoir, physical propertysealing reservoir and fault-related sand lens reservoirs. In the plane, oil and gas distributein the central of area and enrich between Hongdi107well area and Hongcan2well area.In the vertical profile, hydrocarbons are enriched in the Ⅱ and Ⅲ reservoir groups.Tectonic and sedimentation dominated the formation of lithologic traps by controlling thedistribution and geometry of sand bodies. Sedimentary microfacies controlled the porosityand permeability of sandstone, which is the foundation for the accumulation of lithologicreservoirs. The conduit role of fault is also significant for hydrocarbon accumulation. In aword, the integration of tectonic and sedimentation is the premise and foundation while thevalidity of migration networks is key part for hydrocarbon accumulation. In considerationof oil source, trap types and migration networks, we summarized five models ofhydrocarbon migration and accumulation in this area （1） monocline self-source and directmigration model for sand lens reservoirs in fault-nose alarpart;（2） monocline distal sourcemigration model for updip pinch-out sandstone reservoirs in fault-nose alarpart;（3）monocline distal source and fault migration model for low porosity and permeabilityreservoirs in fault-nose alarpart;（4） monocline distal source and fault migration model forsand lens reservoirs in fault-nose alarpart and （5） distal source migration and accumulationmode for structural-lithologic reservoirs in central fault nose. Finally, we evaluated thepotential of Lithologic reservoirs in preferential areas and pointed out the prior play andtarget for further hydrocarbon exploration.