Sedimentary Facies Of The Fourth Member Of The Alshan Formation In The Southern Area Of ​​the Eran Basin

Hanan area locates in the east of A’nan Sag in Erlian Basin. Numbers of oil reservoirs were found in different horizons. The fourth member of A’ershan Formation is one of the major producing horizons in Hanan area. The broken fault block in work area has led to great difficulties in stratigraphic classification and correlation as well as reservoir prediction. Therefore, the enhanced study on sedimentary facies will be undoubtedly of vital importance to predicting reservoir sand prediction and increasing reserves and output of oil field. Based on core observation and description as well as test data analysis, this paper performs a detailed analysis on the distribution and evolution characteristics of sedimentary facies as well as the relationship between sedimentary microfacies and reservoir development in the fourth section of A’ershan Formation.1. Under the control of marker bed and sedimentary cycle, the fourth member of A’ershan Formation is split into three sand groups from the bottom up, including AIupper, AImiddle, and AIlower. Due to denudation, these three sand groups witness a decline in distribution areas in a descending order. They are characterized by large thickness in the middle but small thickness in the south and north as well as the alternation between thick part and think part in the east and west. Formation thickness changes significantly under the obvious influence of synsedimentary fault and palaeohigh.2. A comprehensive judgment is made in combination with ancient geomorphological features according to petrographic composition and textural maturity. It is found that the sediments in the fourth section of A’ershan Formation are derived from Menggulin Bulge, Sunite Uplift and Shayinwusu Bulge.3. The fourth member of A’ershan Formation is the sedimentary system of coastal subsea fan-fan delta-lacustrine facies, which can be divided into6subfacies and microfacies. Coastal subsea fan can be split into three subfacies including fat root, middle fan part and fan end. The developmental microfacies include main streams in fan root and microfacies between main streams, braided streams in middle fan part and microfacies between braided streams, and mud microfacies. Fan delta consists of delta front and frontal fan delta subfacies. The fan delta plain subfacies are poorly developed. Inside the fan delta front, the underwater distributary, sheet sand and inter-distributary bay subfacies are developed. The fan pro-delta is mainly comprised of fan pro-delta mug. Additionally, in the work area, there also are shore-shallow lake subfacies, including beach and bar as well as shore-shallow lake microfacies.4. According to the statistics about large amounts of physical property data, underwater distributary microfacies are most favorable for the storage of facies belt, followed by sheet sand and then by braided stream, and beach and bar.