Study Of Sequence Stratigraphic And Fine Sedimentary Facies Characteristics Of Changxing Formation In Jiannan Area

In recent years, with the improvement of exploration degree of the Permian Changxing Formation reef-bank reservoir in eastern Hubei-northeastern Sichuan, interest in reefs-banks as oil-gas reservoirs has been growing for lots of reefs-banks revealed in Puguang, Longgang, and Yuanba area. Previous researches demonstrated that reservoirs in reef-bank strata of the Changxing Formation were surrounded around "Kaijiang-Liangping continental shelf". Predecessors debated on sedimentary facies distribution of the Changxing Formation in eastern Hubei-northeastern Sichuan, displayed in extension orientation and distribution boundary, which showed great restraint in further study of reef-bank facies and reservoir distribution, largely declining exploration and development effect of reef-bank reservoirs. Therefore, this paper takes Jiannan area of Shizhu synclinorium center in eastern Sichuan as research target, carrying out sequence stratigraphic and sedimentary facies study of the Changxing Formation in Jiannan area, not only having important theoretical significance, but also pratical significance.Based on field profiles, cores and logging-drilling, combing with regional geological background and predecessors’research fruits, guided by Vail classic sequence stratigraphic and carbonate rock sedimentology theories, sequence stratigraphic and sedimentary facies of the Changxing Formation in Jiannan area are studied in detail. Five conclusions are acquired:(1)Based on core observation and laboratory microscope petrographic thin section identification, lithology combination regulars were analysed. In this study, Changxing Formation was divided into two members, the first and second member of the Changxing Formation respectively. The bottom of the first member was mainly composed of micrite limestones and siliceous limestones in low energy; the middle and upper mainly consisted of bioclastic limestones/dolostones, reef limestones/dolostones and crystal powder and finely crystalline limestones/dolostones. The second thickness is thinner than the first, lithology mainly being clastizoic limestones/dolostones, little micrite and crystal powder limestones. Reef-bank mianly occurred in the middle and upper of the first member.(2)Through critical sequence boundaries identification such as lithology-lithofacies change, log response and seismic profiles, Changxing Formation was divided into2type II third-level sequences (Sq1, Sq2). Controlled by third-level sequence, combining with fouth-level sequence subdivision criteria, Changxing Formation was furtherly divided into5fouth-level sequences (sq1, sq2, sq3, sq4, sq5). Only transgressive system tract (TST) and high system tract (HST) were developed and conserved in each sequence, and HST was thicker than TST, some fouth-level sequence didn’t develop TST. Sequence stratigraphic framework of the Changxing Formation was established. (3)Two depositional models were developed in the Changxing Formation in Jiannan area, selectively "gentle slope model" and "platform model". The early stage of the Changxing Formation inherited depositional pattern of the Longtan Formation, showing "gentle slope model", developing carbonate gentle slope facies. In the middle and late stage, energetic reef-bank deposits occurred in northern and southern of Jiannan area resulted from extensional movement of Hercynian and fracturing of syndeposit faults, showing "platform model", developing open platform, platform edge reef-bank, platform front slope and continental shelf facies.(4)According to geometric external form, amplitude and continuance, typical sedimentary facies-seismic facies idenfication signs are established:reef show moundy external form and leggy and indiscrimination reflection in internal; bank show dominant amplitude, continuance, sub-parallel and miscorrelation phase; platform front slope show middle-dominant amplitude, middle continuance, downlap to strong shaft bottom; continental shelf show weak amplitude and blank reflection.(5)In this paper, fouth-level sequence system tract was selected as compilation unit, sedimentary facies distribution figures in sequence framework was established, and planar distribution and vertical evolution characteristics of the Changxing Formation sedimentary facies was analysized in detail in Jiannan area. sql-TST inherited the depositional pattern of former stage, being "gentle slope" deposition, reef-bank were not developed; sql-HST, platform was formed, some platform edge reef-bank were developed, laying the foundation of latterly extensive development of reef-bank; sq2-HST, reef-bank were extensively developed, widely distributed in two lateral of continental shelf; sq3-HST, reef-bank development continued, but became feeble and die in the evening, the scale of which is smaller than the former; sq4-TST, sea level raised again, resulting in platform edge shrink, developing little reef-bank; sq4-HST, sea level dropped again, depositional pattern changed, continental shelf shrinking towards southern, reef-bank developed with large scale again; sq5-HST inherited depositional pattern of sq4-HST, bank continued developing.This study not only re-delineats "Kaijiang-Liangping continental shelf", finely describes the Changxing Formarion reef-bank distribution in Jiannan area, deepening reservoir-sedimentology theory connotation of marine carbonate rocks, but also has a significant guideness in distribution prediction and oil-gas exploratory development of reef-bank reservoirs in the study area.