Study On Sequence Stratigraphy Of Middle Triassic Leikoupo Formation In The Western Of Sichuan Basin

In West Sichuan, hydrocarbon exploration and research in the previous stage mainly focused on middle-shallow continental reservoirs, and has acquired substantial achievements. However, the exploration of marine reservoirs in West Sichuan relatively lagged behind. Lacking of adequate research has become the main factor restraining the exploration of marine reservior in this region. Although the hydrocarbon exploration within the Middle Triassic Leikoupo Formation in Sichuan basin has discovered only two large-medium scaled gas fields (Zhongba gas field and Moxi gas field) so far, there were active oil and gas showings during well-drilling within Leikoupo Formation in the recent years, illustrating a bright prospect of exploration.Being guided by carbonate rock sedimentological and sequence stratigraphical theories, the study aims to trace and determine the regional sequence boundary of the Middle Triassic Leikoupo Formation in West Sichuan, based on the observations on outcrop profiles and description of under-bore cores, in combination with comprehensive elaboration on seismic and logging data; and through single-well and link-wells tratigraphic sequence analysis, to establish the stratigraphic sequence framework for this region. Through in-depth study on the carbonate sedimentary systems and other micro-facies characteristics, the pattern of regional sedimentary facies was determined. Based on this pattern, the lithofacies paleogeographically spatio-temporal distributions of the main sequences within the framework were also studied, to illuminate the spatial distribution patterns of the key reservoir stratas and to point out the favorable reservoir facies belts, thus, providing theoretic evidence for the prediction of reservoir distribution and the exploration of large-scaled oil-gas fields in this region.The results of field investigation and under-bore core observation showed that there were three genetic types for the sequence boundary of Triassic marine reservoirs: consequentially, the uplift erosional unconformity and the local exposure erosional unconformity and the lithology-lithfacies transitional boundary. By using present filed seismic profile, under-bore core observation, loggin and seismic data and etc., and employing the classic sequence stratigrahpic theories, Leikoupo formation was divided into one Grade Ⅱ sequence(SSQ1) and four Grade Ⅲ sequances (SQ1-SQ4). Each of the sequences was further divided into the transgressive system tracts (TST) and the high-stand systems tract(HST). On this basis, a cross-sectional comparison on the sequence of Leikoupo formation was conducted, to establish the sequence stratigraphic framework of Leikoupo formation in West Sichuan.Through analysis on the various facies marks, such as rock type, sedimentary structure and etc., the Middle Triassic Leikoupo formation was classified as a epicontinental-type terrace system. Its sedimentary facies were categorized into two main types:the restricted platform facies and the evaporate platform facies. Partial region was open platform facies. The main sub-types were:lagoon, tidal flat, exposed shallow, evaporate flat, evaporate lake, saline lake and etc. By elaborating the characteristics of the various sedimentary facies, the pattern of sedimentary facies of Leikoupo formation was determined, i.e., the epicontinental-type terrace pattern.From the above, the study further analyzed the characteristis of sedimentary facies and lithofacies distribution within the sequence framework. This paper was the first to use systems tract as the mapping unit, compiling paleogeographic maps for the lithofacies of various reservoir strata within Leikoupo formation. According to the paleogeographic characteristics of lithofacies at the various sequences, in combination with the investigation on conformation patterns of the previous and contemporaneous tectonics maps for the subject area, the paleogeomorphological evolution pattern of this region was elaborated. During SQ1-SQ4period, Leikoupo formation generally depicted a trend of "shallow in south and east, deep in north and west", illustrating a paleogeomorphological pattern of "lower in north and west, higher in east and south". The study predicted an existing of fossil karst slope and plateau at the top Leikoupo formation, which was advantageous of the development of fossil karst reservoir. Considering property data characteristics for reservoirs at different periods, a comprehensive investigation on the reservoir strata within the subject area was performed, to draft a prediction that the sequences worthy of emphasized exploration within Leikoupo formation were:SQ1, SQ3and SQ4.