Seismic Sedimentology Of Deltaic Depositional Systems And Characterization Of Delta-related Lithologic Reservoirs

Guided by the theories of sequence stratigraphy, sedimentary lithology, petroleum geology and geophysics, the study focused on the seismic sedimentology of Olmos Formation in Maverick Basin, South Texas, USA and the Permian, Triassic and Jurassic strata in Wu-xia Area, Junggar Basin. The lithologic characteristics, sedimentary structures, facies distributions and sedimentary history of deltaic depostional systems were concluded using 3D seismic data, core data, log data and others, consequently, the characterizations and control factors of delta-related lithologic reservoirs were proposed.There are several sets of source rocks, reservoirs and seal rocks in up Cretaceous strata of Maverick Basin and Permian, Triassic and Jurassic strata of Wu-xia Area, with numerous faults and unconformity surfaces developing widely. Thus, the conditions of oil and gas generation, migration and accumulation in the study areas were favorable.The up Cretaceous strata of Maverick Basin were divided into 8 third-order sequences or 17 parasequences in 6 sequences sets, and the depositional systems in the up Cretaceous strata of Maverick Basin include wave-dominated barrier bar depositional system, wave-dominated delta depositional system, wave-influenced delta depositional system and regressive shoreface depositional system. In Wu-xia Area, 24 sequence boundaries, 23 third-order sequences, 7 second-order sequences and 1 first-order sequences were identified in Permian, Triassic and Jurassic strata. Lacustrine facies depositions were dominant in early Permian, and then fan delta facies in middle Permian. Wu-xia Area was in a lacustrine transgerssive process. The lacustrine surface was in a rise-descend-rise process in Triassic, fluvial fan-braided river depositional system was dominant in JSQ1, JSQ2 and JSQ3, with fan delta and delta-lacustrine depositional system in JSQ4 and fluvial fan-braided river depositional system in JSQ6.Seismic sedimentology has a good performance in mapping depositional system in Maverick Basin and Wu-xia Area. Seismic amplitude in 90°-phase data and lithology correlate well in the study strata, and stratal slices made from the 3-D seismic volume reveal high-resolution sediment dispersal patterns and associated systems tracts on relative geologic time surfaces. The Olmos formation is characterized by deltaic systems, from which five types of depositional systems were identified: fluvial plain, deltaic plain, deltaic-front, prodelta and incised valleys. Meanwhile, the Baikouquan formation and Kelamayi formation in Wu-xia Area are characterized by terrestrial deltaic depositional systems and fan deltaic depositional systems.With cross correlations and core descriptions, distributary channel, levee, crevasse splay, swale, lagoon, interchannel sediment, shoal, barrier bar, barrier and prodelta were recognized in Olmos Formation of Maverick Basin, and Catarina deltaic depositional system, Rocky Creek barrier and strandplain depositional system, Big Foot depositional system were identified, in which 5 progradation cycles developed: unit A, unit B, unit C and F, unit D and G, unit E and H. In Wu-xia Area, the terrestrial deltaic facies contain fan deltaic facies and deltaic facies. Using cores and logs data, some types of sedimentary facies were identified: fan deltaic plain, distributary channel and interchannel sediment, deltaic front bar, deltaic distributary channel, braided river channel, swale and lacustrine sediment.Based on the studies of sequence stratigraphy, seismic sedimentology and sedimentary lithology, the depositional models and sedimentary histories were concluded. Olmos Formation deposited mainly in the western sedimentary center in Maverick Basin along Rio Grande Embayment during the early depositional stage, and the depositional units A, B and C in Catarina Deltaic system were formed. In the period of unit C deposition, sediments entered the eastern sedimentary center, and the deposition in eastern sedimentary center was dominant in Maverick Basin during the middle and late stages of Olmos Formation deposition, while Big Foot depositional system was forming. At the same time, Rocky Creek barrier and strandplain depositional system formed in the the western sedimentary center. Wu-xia Area was a foreland basin in early Permian, and transferred to a depression basin in late Permian. The Permian strata are characterized in fan delta-fluvial fan-lacustrine depositional systems, the Triassic strata are characterized in fan delta-fluvial fan-delta-fluvial plain depositional systems, and the Jurassic trata are characterized in fluvial fan-fluvial plain-delta-lacustrine depositional systems.In terms of the sedimentary facies types, structural characteristics and trap types, the reservoirs of Olmos formation in Maverick Basin were divided into 7 delta-related types: updip unconformity-stratigraphic reservoirs, Charlotte Fault Zone fault-stratigraphic reservoirs, updip structural reservoirs, volcanic mound reservoirs, deltaic and shore-zone sandstone structural reservoirs, downdip deltaic and shelf reservoirs, shelf edge reservoirs, etc. In Wu-xia Area, the delta-related reservoir types contain structural reservoirs, stratigraphic reservoirs, thithologic reservoirs and complex reservoirs. The main control factors of oil and gas immigration and accumulation include deltaic depositional system distribution, source rock distribution, unconformity characteristics, fault characteristics and volcanic intrusion. Seeing evaluation of typical traps, the reservoirs where deltaic depositonal systems exist have good accumulation conditions, better sandbody connectivities and good show of oil and gas, and can be the key targets of potential lithologic oil and gas play. According to the sources of oil and gas, migration pathways and directions, and the reservoir-forming periods, four types of oil and gas accumulation models are concluded in Wu-xia area: (1) near source- lateral sand body and unconformity migration-early accumulation models; (2) near source-vertical fault migration-multiple phase accumulation models; (3) long distance migration- multiple migration pathways- multiple phase accumulation models; (4) long distance step-shaped migration- multiple migration pathways- late accumulation models.