Research On Sedimentary Characteristics Of TheYanchang Formation In Fuxian Area, Ordos Basin

Ordos Basin has a promising prospect of oil and gas exploration with rich in petroleum and gas resource, where many Petroleum systems have been found from Mesozoic to Paleozoic. Several wells in Fuxian area have industrial oil by oil testing and some oil fields are found early or later near the area. These cases show that Fuxian area has great potential in exploration and development. However, the special surface conditions and geological characteristics of Ordos Basin bring more difficulties for development. How to further enhance the level of comprehensive geological study, enhance the geological understanding,solve technical bottlenecks of exploration are the key to having a break in oil and gas exploration.Stratigraphic division of the Yanchang Formation and the Yanchang Formation provenance analysis are base on comprehensive study of field outcrops surveying ,the core observation,thin section identification and logging analysis. A variety of sedimentary facies are identified by the rock characteristics analysis, paleontology characteristics and logging facies. On the basis of that,through studying the single well sedimentary facies and profile facies with analysis and interpretation log curves, the sedimentary facies and sand area of space distribution are clarified. At last, the paper establishes the facies model and predicts the space distribution of favorable facies belt.Main types of sandstone from the Chang-8 member to the Chang-2 member are arkose and lithic arkose, with the main characteristics of high content of feldspar in Fuxian area. Mainly sandstone grain size is fine grain sandstone and siltstone, distribution of the week. On the base of paleocurrent analysis and sandstone type and framework mineral analysis, source direction of the Chang 8 deposition period is mainly Northeast in Fuxian. South source exist at the same time, but it limited impact on the South of Fuxian area; Northeast and North provenance mainly exsit in the Chang 6 deposition period, without source south; The main source direction of the Chang 2 +3 deposition period is the Northeast; Episodic Indo-China movement of the Chang 7– Chang 6 deposition period results in changes in sedimentary framework, thus affects the source direction.Comprehensive using of the rock characteristics, sedimentary structure types characteristics and minerals types and combination of fossils,fossil sites with logging facies, the paper identifies four major categories of fluvial sedimentary system, delta sedimentary system, lacustrine sedimentary system and lake turbidite fan in the Yanchang Formation in Fuxian. Delta sedimentary system dominated, lake turbidite fan just distributes in the Chang 7 member and Chang 6 member in southwest of the area. The sedimentary microfacies of delta front ,Mainly developed in the Chang 9 - the Chang 6 member, can be determined to be underwater distributary channel, underwater interdistributary, mouth bar, distal bar, sheet sand; delta plain, mainly developed in the Chang 4+5 - the Chang 1 period, include branch channel, inter branch channel , the crevasse splay microfacies.According to the core observation, logging analysis to combined with sedimentary background research, that turbidite deposition develops in the southwest and west of Fuxian area in the Chang 7,the Chang 6 period, which is rich in sedimentary deformation structure. Its sedimentary characteristics are somewhat different form that of the north and northeast, the lack of structure in shallow water or alluvial genesis. The study considers Chang 7 and Chang 6 member develop mixed genesis lake turbidite fan in the west of Fuxian area, mainly delta slump-type. Its subfacies are divided into middle fan (microfacies as turbidite channels and inter-turbidite channels), outer fan (microfacies is out fan mud), with inner fan under developed. Slip blocks of Delta front supply relatively limited material, so turbidite fan is small, no obvious supply channel, and sediments are closer to the lake shore, mostly as thin turbidites. Finally, based on the analysis front, this paper establishes shallow delta sedimentary model and lake turbidite fan sedimentary modle for Fuxian Yanchang Formation, and predicts favorable facies belt and block for Chang 8,Chang 7,Chang 62,Chang 32.