Sedimentary Characteristics Of Lacustrine Carbonates In The Upper Fourth Member Of Shahejie Formation ?Es₄^s?,Western Dongying Depression

Lacustrine carbonate rocks are very important oil&gas plays in ancient lake basins,which are complicated but prospective and therefore become the difficulty and hot-point in the research field of carbonates across the world.For the western Dongying depression,carbonate sediments are widely developed in the upper fourth member of Shahejie Formation?Es₄^s?,but the further exploration has been greatly restricted due to limitated understandings of sedimentary characteristics.Based on the analysis of seismic profiles,well loggings,cores,thin-sections and geochemical measurements,this paper studies sequence stratigraphy,sedimentary characteristics and evolution of carbonate rocks in western Dongying Depression.Then,a sedimentary model of wind/wave-controlled semi-isolated carbonate platform is proposed,and the main controlling factors on sedimentary process and formation mechanisms are discussed for further understanding.According to texture-genetic characteristics,carbonate rocks of Western Dongying are divided into eight types including framestone,bindstone,rudstone,grainstone,packstone,wackstone,micritic carbonate and mixed carbonate-silicilastic rocks.These rock types have strong relationships with logging properties to establish an uniform lithological identification template,which promotes the lithological identification accuracy in uncored wells and aids the next analyses on sequence and sedimentary facies.Es₄^s was considered as a third-order sequence,composed of lowstand?LST?,transgressive?TST?and highstand system tracts?HST?.According to cross-sections,it usually only developed TST and HST without LST above fault-induced break belts,meanwhile the carbonate strata are mainly developed in HST.Based on topography and water depth,four types of parasequence are identified as shallow ramp,deep ramp,steep slope and deep sag.Combined with the second interpretation of lithology and the stacking pattern of parasequences,the HST are divided into six to seven parasequences and two parasequence sets in the vertical,which constitute the five-order high-resolution sequence framework in the carbonate strata.The sedimentary facies are recognized as a semi-isolated carbonate platform due to three sags surrounding on three sides.Eight subfacies are recognized as marginal reef-bank,intraplatform reef-bank,shoal,inter-shoal,platform topographic low,ramp,slope and semi-deep to deep lake,of which the two reef-banks are NE-oriented in spatial distribution to reflect the significant effect of southeast winds.In addition,the sedimentary evolution is studied in the high-resolution sequence stratigraphic framework.At the earlier stage of HST,it was dominated by shoals and intershoals with few patch reefs,while at the later stage it showed larger-scale reef-bank and shoal composite that characterized by obvious enlarging shoals and more developed patch reefs.Comprehensive sedimentary model of the Es₄^s carbonates of western Dongying Depression is a wave-dominated semi-isolated carbonate platform,whose formation process was greatly affected by paleo-geomorphology,water depth,wind field,climate,salinity and provenance.However,the dominated three controlling factors on sedimentary systems are the local paleogeomorphology,lake level changes and wind/wave reconstruction:1)the restored paleogeomorphology showing a macroscopical underwater uplift and various internal microtopographies that control the corresponding subfacies types and their developments;2)the overall shallowing water depth and lake level changes in five-order cycles control the development of patch-reefs and the evolution of sedimentary system;3)the prevailing southeast winds and waves produce different hydrodynamics in the platform to promote the formation of two rows of reef-bank zones.The intense wind and wave within windward belts could destroy earlier patch reefs to promote the reconstruction of large-scale distributed reef-banks and shoals in the study area.