Research On Reservoir Sedimentology In Southern Sag Of Wuliyastai Depression In Erlian Basin

The Wuliyasitai depression, located at the northeast of Erlian basin, includes three sags of southern, middle, and northern. Present hydrocarbon exploration was mostly concentrated at the southern sag. By means of core observation, cast thin section analysis, X-ray diffraction analysis, granularity analysis, SEM analysis, capillary pressure analysis, synthetically using sequence stratigraphy, sedimentology, reservoir geology and reservoir evaluation, the sedimentary reservoir in southern sag of Wuliyasitai depression was studied in detail. The following results and recognitions have been obtained.1) Stratum in the research region, was divided into 6 three-order sequences, namely Jurasic sequence, Arshan Formation sequence, Teng 1 lower member sequence, Teng 1 upper member sequence, Teng 2 member sequence and Saihanta Formation sequence. The Arshan Formation sequence (including three four-order sequences) and Teng 1 lower member sequence (including two four-order sequences) are emphasized in this investigation.2) Lacustrine sediment, sublacustrine fan and fan delta were identified in the research region in which sub-lacustrine fan and fan delta were primary sedimentary facies. Lacustrine comprises coastal-shallow lake and semi-deep lake; sub-lacustrine fan comprises feeding channel, inner-, middle- and outer-fan sub-facies; fan delta comprises plain and frontal sediment sub-facies. Lacustrine sediment developed in Arshan Formation and Teng 1 lower member; fan delta developed in Arshan Formation and later stage of Teng 1 lower member; sub-lacustrine fan developed in early stage of Teng 1 lower member. The frontal branch waterway of fan delta and braided channel in middle fan of sub-lacustrine fan are advantageous sedimentary facies, however the frontal wedgelike sand of fan delta and main channel in inner fan of sub-lacustrine fan next.3) Rock types in the research region are conglomerate, glutinite, lithic arkose, and feldspar lithic sandstone. Sandstone diagenetic types include mechanical compaction, cementation, metasomatism and denudation during which compaction and cementation reduced porosity and permeability of reservoir sandstone, however denudation did the other way round. Diagenetic facies involve carbonate cementation and metasomatism, authigenetic kaolinite cementation, zeolite cementation and quartz secondary outgrowth cementation during which authigenetic kaolinite cementation belt which accords with the secondary porosity upgrowth belt was provided with favorable poroperm characteristics, whereas carbonate cementation and metasomatism, zeolite cementation and quartz secondary outgrowth cementation belt opposite. Diagenetic intensity is at the early diagenesis stage B to middle diagenesis stage A.4) Reservoir pore mainly contains intergranular pore, intergranular and intragranular dissolved pores. Three secondary pore development belts exist in the study area, holding a good corresponding depth with diagenetic stage. Two genetic mechanisms of secondary pore exist in sandstone reservoir: organic and carbonic acid produced from organic material thermal evolution and acidic fluid produced by clay mineral transform.5) Three pore configuration types of mesopore mid-throat homogeneous type, mid to fine pore thin throat mid-homogeneous type, and fine to micro pore and throat unhomogeneous type exist. Three types of reserviors in the research region were divided as Types I, II, and III. With mesopore mid-throat homogeneous type, frontal branch waterway of fan delta and braided channel in middle fan of sub-lacustrine fan are favorable reservoir, Type I; frontal wedgelike sand of fan delta and main channel in inner fan of sub-lacustrine fan were divided as Types II and III.