Abstract
Wuerxun depression is one of the depressions with great exploration potential in Hailaer Basin and has submitted large-scale reserves. At present, it has entered the stage of fine exploration, and the exploration object has changed from structural reservoir to lithologic reservoir exploration. The remaining targets are mainly concentrated in the trough and surrounding areas, with strong concealment and difficult to identify. Fine identification of sand bodies, genesis, types and distribution of sedimentary fans are one of the key factors restricting oil and gas exploration. Based on core observation and genetic mechanism, three sedimentary facies models of Braided River Delta, fan delta and sublacustrine fan are established. In this paper, the method of “sequence control, cycle correlation and hierarchical closure” is used to fine characterize the fan delta sedimentary system in this area, which lays a foundation for the study of sedimentary microfacies of subdivision layers, optimization of lithologic reservoir targets and guidance of oil and gas exploration deployment.
Highlights
Wuerxun depression is one of the main oil-rich sags in Hailaer basin, with an area of 1850 km[2] and great resource potential
It is usually formed on the steep side of short axis of depression, with large terrain elevation difference and close to provenance. It is a coarse-grained sedimentary system generally developed under the background of growth fault. It reflects the lithologic characteristics of shallow water and is mainly formed by traction flow.In the fan delta plain, massive beddings and conglomerates are developed, and the scour surface, progressive bedding and cross bedding are developed in the front of the fan delta; parallel bedding and deformation structures are developed in the front fan delta; and the typical three-layer structure is found in the sedimentary sequence
The front delta is composed of grey mudstone and argillaceous siltstone with reverse grain sequence; the front edge is positive grain sequence glutenite intercalated with gray mudstone and siltstone; the plain is positive grain sequence glutenite and purplish red and gray mudstone(Fig. 1)
Summary
Fan delta is a depositional system directly pushed into Lake Basin by alluvial fan. It is usually formed on the steep side of short axis of depression, with large terrain elevation difference and close to provenance. 2.2 Fan delta front subfacies Fan delta front deposit is the main body of fan delta deposit, and its lithology is mainly gray, gray black conglomerate, conglomerate, coarse sandstone and thin gray mudstone. It has the characteristics of normal cycle with coarse in the lower part and fine in the upper part. 2.3 Fan delta subfacies The front fan delta deposits are developed at the farthest end of the offshore fan delta, and it is difficult for the provenance supply to reach the area directly It is mainly composed of suspended still water sediments, mainly composed of interbedded gray green, gray black mudstone and argillaceous siltstone. (b The fault-step slope break zone and sedimentary model of the South Subconcave steep slope of Wuerxun)
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