The Research Of The Rule Of Depositional System And Distribution Of Lower Silurian Xiaoheba Formation In Southeast Sichuan

The lower Silurian Xiaoheba formation(Shiniulan formation)mainly distributed over the gas accumulation zone in southeastern Sichuan Basin. we found indication of oil and gas in well Jianshen 1, Paleooil reservoir of Shiniulan formation in Xishui ,south Sichuan-north Guizhou.Longmaxi formation consist of black mudstone,shale as the source rocks,which was covered by sandstone of Xiaoheba formation,and thick mudstone,shale of Hanjiadian formation as the seal rock. Xiaoheba formation prepared wide-area reservation,moderate burial,advantageous geology setting for becoming a good reservoir.In short Silurian of lower Palaeozoic had good conditions on the source-reservoir-seal associations.Under this advantageous exploration survey,the question about whether had effective reservoir or not must be solved immediately.This article based on sedimentology, sequence stratigraphy as theoretical direction, outcrop,data of well drilling and well logging, test analysis,combined with the result of research of predecessors.Using the provenance analysis and research of depositional system, sequence stratigraphy and Sequence-based lithofacies and paleography,in order to discover the rule accumulation and distribution of materials,and main controlling factors, provide theoretical basis for research of distribution of effective reservoirs and source-reservoir-seal associations,etc.This research contains content and results as follow:(1)By the field measurement of palaeocurrent direction,the research of sandstone components, heavy mineral, geochemistry(rare earth elements, trace elements),and the characteristics of macro distribution,in order to reveal that the provenance direction of sandstone of Xiaoheba formation mainly came from 265°～356°,(Jiangnan-Xuefengshan uplift was main provenance). Provenance consist of multi-cycle sedimentary rock,which came from older uplifted active continental margin and passive continental margin. We draw conclusion from the analysis of heavy mineral and geochemistry that the provenance came from the west wing of Xuefengshan uplift(Guzhang,Zhijiang,Yuanling,Huaihua),also involved basic rocks,ultrabasic rocks, basic-intermediate effusive rocks of upper Proterozoic Banxi group. (2)By the research of outcrop,drilling facies,logging facies,etc.There were two depositional system sets,three depositional systems,five sedimentary facies and types of parfacies,microfacies. Paralic sedimentary system sets mainly consisted of delta,which located near Xuefengshan uplift in southeast Sichuan-west Hunan. Marine depositional system sets mainly consisted of shelf, carbonate platforms,which mainly located in south Sichuan-north Guizhou.(3)Based on identified sequence boundaries, systematically divided two third-order sequences,four fourth-order sequences.Then we make comparison of the sequence of key sections and wells,in order to construct relative sequence stratigraphic framework of the Xiaoheba formation(Shiniulan formation)of lower Silurian in southeast Sichuan.(4)By the research of evolution of depositional system in the sequence stratigraphic framework,we make clear the rule of vertical distribution of depositional system,research shows that the deposition sequence of material of Xiaoheba formation, mainly developed in high system tract and transgressive systems tract. By mapping the map of lithofacies palaeogeography,in order to reveal the characteristics of spread of horizontal sedimentary facies. Further revealed the material of Xiaoheba formati on accumulation distribution is mainly controlled by tectonic activity, the ancient landscape, source supply, climate and sea level change and other factors.Based on above research,we can get the theoretical basis of favorable reservoirs of Silurian in southeast Sichuan.