Study Of Sequence Stratigraphy And Sedimentary Facies Of Triassic System Of AKEKULE Area In Tarim Basin

Recent years, petroleum explorations of Triassic in Akekule area have made great progress. But studies of sequence division and sedimentary facies are lag. The sedimentary konwledge and sequence division of former researcher could't correspond with exploration needs.Based on former studies, the paper utilized observations of outcrop section of Kuche river and cores of exploratory wells and appraisal well that were as much as 50,integrated with some analysis and test data, to study the sedimentary sequences and facies of Akekule area in detail. The stratigraphy could be divided into six third-order sequences, which was sequence SQ1, SQ2, SQ3, SQ4, SQ5 and SQ6 from lower to upper lay. The sequence SQ1 distributed in Ketuer formation of lower Triassic. Sequences of SQ2, SQ3, SQ4 distributed in Akekule formation of middle Triassic, and sequence SQ5 and SQ6 distributed in Halahatang formation of upper Triassic. The boundary of these sequences all were typesâ… . These sequence boundaries could be recognized clearly by cores, outcrop and seismic data. Every sequence could be divided lowstand system tract (LST), trangressive system tract (TST) and highstand system tract (HST), but development conditions of sequences were various. The TST and HST developed well, but LST didn't develop in sequence SQ1. The LST developed well, but TST and HST didn't develop in sequence SQ2. The LST and TST developed well, but HST didn't develop in sequence SQ3. all system tracts developed well in sequence SQ4. The HST didn't develop in sequence SQ5. The LST and HST developed well, but TST didn't develop sequence SQ6. The reasons that caused system tracts developing unevenly were that exposure and erosion arose by base level fell.Though the correlations of sequences of well tie, plane distribution feature of systems tracts of all sequences was analyzed. The system tracts of sequences distributed in southwest of study area developed well, and erosion intensity was lower. But the erosion of lay distributed in northern area was intense, and lost lots of system tracts. Based on correlation of system tracts of sequences, the change feature of water depth, sedimentary source and lacustrine level could be analyzed clearly. And framework and model of sequences could be summaried.On the based of integration of core, outcrop and kinds of analysis and test data, sedimentary types were analyzed. Based on division of sedimentary system of single well, plane distributions of sedimentary systems were analyzed through sedimentary systems correlation of tie well. The types of sedimentary system comprised incised river valley filling, lacustrine, incised canyon, sublacustrine fan, delta and alluvial fan. Among them, the incised canyon and sublacustrine fan were found for the first time in Akekule area. Recently, on the based of this breakthrough, wells of AT1, AT2, GP4 and JN5 had product lots of petroleum and created great value. Incised river valley mainly distributed in LST of sequences, and it was braided river in nature, and it also was the main facies of all oil members. Shallow lacustrine distributed widely in upper portion of LST and HST. Delta distributed in HST mostly. But HST was eroded mostly during base level fell, and HST was remained locally in area.Analysis of physical property, mercury penetration and cast thin section data, the physical property of all main sand bodies (LST of all sequences) was better, and average porosity was about as much as 20% and connectedness of pore was good. Diagenesis was also favorable for formation and preservation of pore. According to criterion of reservoir evaluation of Tarim, the reservoir of Triassic mostly belonged typeâ… toâ…¡. So the reservoir was one of the best reservoirs.Although the reservoir of Triassic had good preservation property, main sand bodies of all oil members were interpreted to be water lay. The main reason was that large and effective structures were absent in main sand body in Triassic. The top structural figures of main sand body of all oil members (LST sand body of sequences SQ2, SQ3, SQ4, SQ5, SQ6) identified that study area presented a monocline southwards. Even local microstructure couldn't form large reservoir. Recently, lithologic trap formed by some thin sand body produced petroleum, such as wells THN1, S110, S118 and S113. So the non-structural trap that was paid little attention to ever became research focus again. The non-structural trap of Triassic comprised lithologic pinchout trapand lensing lithologic trap. lithologic pinchout trap distributed in LST and HST, but lensing lithologic trap distributed mainly in HST.