Research Of Sedimentary Facies Of Sangonghe Formation In Cainan Oilfield

Cainan oilfield is in the east of Junggar Basin, which is located in the north east of about140km from Urumqi, the capital city of Xinjiang Uygur Autonomous Region and due west about60km from Huoshaoshan oilfields. Its east-west width is about17km. North-south width is about7km. The average ground elevation is690m.It is a typical inland basin in arid desert oilfields.Cainan oilfield is at the intersection part of three secondary tectonic units, the northern sag of Dongdaohaizi area, the sag of Wucaiwan and Baijiahai arch. Baijiahai arch is a large-scale southwest nose structure, which is based on the Paleozoic uplift. There is partial lacuna in the Carboniferous strata. Permian was a period of erosion that began to deposit. The upper Permian strata was denuded in the Indo-China movement, while the Cainan anticline was to take shape. Jurassic strata in this area sedimentsed completely and distributed generally. It is a stable layer, whose thickness is1100m～1500m. It is a Pan-relaxation basin deposition in the context of release stress, while the lake expanded far beyond the present basin boundary. During the period of the later Badaowan and Sangonghe, there were two clear lake transgression, especially in the Sangonghe deposition, which had deep water and higher abundance of organic matter. Xishanyao formation was a gradual deposition of a lake regression, which was based on the Sangonghe lacustrine deposits. The area rose again in the middle Yanshan movement, while the Cainan anticline, which had flat structure and west wing angle3°～4°, east wing angle4°～5°, and produced many NE and EW trending normal faults, developed further. Well block Cai8-Cai24is located in the eastern CaiNan oilfield. The structure is a secondary structure in CaiNan anticline structure, which is located in western binding site of the eastern Baijiahai salient and Wucaiwan depression.From base to top, the strata of Cainan oilfield are Quaternary(Q), Neogene(N), Paleogene(E), the Cretaceous Tugulu group (K1tg), the Jurassic Shishugou group (J2-3sh), the Xishanyao formation(J2x), the Sangonghe formation(J1s), the Badaowan formation (Jib), the Triassic Xiaoquangou group (T2-3xq), the Shaofanggou formation(Tis), the Jiucaiyuanzi formation (T1j), the Permian Wutonggou formation (P3wt), the Carboniferous Batamayineishan formation (C2b), the Dishuiquan formation (C1d), the Tamugang formation(C1t). The sandstone of the Jurassic Sangonghe Formation was developed, as the main purpose of layer.The Cainan operation area and other units have been carried the work on base and production. They have a certain degree of understanding of sedimentary reservoir, reservoir character and so on of Cainan Oilfield, In sedimentary respects, basic geological comprehensive study of the Cainan operation area in2009is especially the most systematic. With the improvement in the level of the J1S reservoir, there is some dispute on types of sedimentary facies and provenance direction.On the basis of a comprehensive study of core, log well, the data of mud logging and testing analysis, taking the high resolution sequence stratigraphic theory as a direction, taking the theory of high-resolution sequence stratigraphy as a direction, the stratigraphic division and correlation has been performed. Combined with macroexamination and micro-analysis, the Jurassic Sangonghe Formation in well block Cai8-Cai24in Cainan oilfield was firstly analyzed systematically. Through this integrated study, we provided geological parameters to reservoir development. The research of results is important to the oil-gas exploration and development in this area.Based on high-resolution sequence stratigraphy, analysis on log data, lithological characteristics show that the bottom-up lithology of J1s in Cainan Oilfield from fine to coarse to fine, which is progradational-retrogradational cycles, reflecting the lake level first decreased and then rising evolution. Jis can be divided into one and half medium-and long-term cycles.J1S2can be further divided into a middle cycle, two short-term cycles. According to the regional comparison, the Jurassic Sangonghe formation in Cainan oilfield can be divided into three members, which are J1S3、J1S2、 J1S1,from top to bottom. The sedimentary thickness of J1S3is about25m-101.7m, and the average is65.7m. The lithology of J1S2is primarily siltstone and fine sandstone, moderate coarse sandstone, and conglomerate. At the top of J1S2is a relatively stable mudstone. It is the main producing reservoir, which is a good reservoir-seal assemblage with J1S3. The sedimentary thickness of J1S2is about4.5m～45.7m, and the stratum that near the east basin margin is eroded. The member can be subdivided into two sand groups, while oil layer is mainly in J2S22. The J1S1is mainly composed of dark gray, silty mudstone, sandy mudstone, fine sandstone, coarse sandstone and some sandy conglomerate. The sedimentary thickness of J1S2is about23m～104m, and the average thickness is65.15m.The formation of eastern edge of the basin is missing.Taking use of core observation, the data of mud logging, the wire line logging and testing analysis, we recognize that Jurassic Sangonghe formation in Cainan oilfield belongs to the deposition of braided-river delta, There are braided-river delta front subfacies, prodelta subfacies and shallow lake subfacies, and seven microfacies, which are underwater distributary channel, channel mouth bar, bay mud between distributary channels, far bar sands, prodelta mud, shallow lake muds and shallow lake sand bars, in the area where we studied. Underwater distributary channel is the main body of delta front. Braided delta front is the most active place of the deposition of braided-river delta, whose deposition is the main body of braided-river delta. Sedimentary structures in braided delta front were developed and construction types are also very rich, such as cross-bedding, parallel bedding and wave-ripple bedding. There are more scour surface and mudstone fragments, whose shape is long strip with edges and corners. This reflects the strong hydrodynamic underwater distributary channel. Prodelta is located in braided river delta front slope to the direction of the lake deeper water zone, which is often associated with the deposition of semi-deep lake. Its lithology is dominated by dark gray mudstone and silty mudstone, with horizontal bedding or massive bedding.The determination of the provenance can help to predict source-reservoir-cap assemblage of the target area. There are various methods for the provenance analysis, such as:description of characteristics of debris, the thickness distribution of conglomerate and sandstone. In this research, we use the color of mud, the thickness distribution of conglomerate and heavy mineral analysis to determine that the provenance comes mainly from northeast and east.From J1S1～J1S3the Sangonghe formation reflects a more complete set of former braided river delta-braided river delta front-former braided river delta. J1S1is alternating deposition of relatively shallow lake-braided river delta, and sand deposition thickness variation is relatively large, the water flows from the North East and East. J1S2is lake regressive delta after fluctuation environment. The distribution of braided-river delta front is the most widely. Provenance direction is the same as J1S1. In the period of J1S3, lake level continued to remain high throughout the area, and underwater distributary channel had been gradually shrinking. There formed alternating deposition of regional shallow lake and former braided river delta, which is mainly shallow lake mud and prodelta mud.