Sedimentary Characteristics And Facies Model Of Middle And Lower Carboniferous Carbonate In The Eastern Caspian Basin

Carboniferous carbonate formation is the main oil-bearing series in the eastern Caspian basin,which is deposited in tropical and subtropical shallow sea environment? Previous studies that the middle and lower Carboniferous series(KT-? reservoir group)is the open platform facies,but there is no clear explanation of platform edge.Due to the lack of drilling data,the sequence division is not detailed and accurate.The previous studies in this area is not enough on the interior formations of KT-? reservoir group,and short of its sedimentary feature and facies mode.Based on investigation of the eastern Caspian basin of geological background and sedimentary environment,using drilling data,logging data and seismic reflection data,this article have divided sequences on single well and cross-wells.The Carboniferous carbonate formation have been divided into five second-order sequences and nine third-order sequences.In the middle and lower carboniferous formations,there is SS1-SS2 second-order sequences and S1-S3 third-order sequences,and 2 complete second-order sedimentary cycle,which indicates the sea level changes.Through recognizing the facies marker of the lithology,slice analysis,well logging,and seismic reflection to determine that the eastern Caspian basin developed in carbonate ramp facies and intertidal zone subfacies,whose sea level lies between the mean high water and mean low water.Then identified four microfacies : the bioclastic bank,melange grain bank,mud flat and interbank subsag.Considered the cross section of sedimentary,paleogeomorphic,mirofacies parameter and the seismic attributes,found that the bank in Bashkir formation is the largest deposition,which lies in north of the study area.As the result of the sea level rising,the bioclastic bank and melange grain bank moved from west to east and the scale decreased,while the zonal distribution shape turn to be sporadic scattered in the mud flat.According to the above sedimentary characteristics,it is concluded that the middle and lower carboniferous formation(KT-? reservoir group)in the study area belong to the carbonate ramp facies model with eastward advancing foreland development and knoll internal structure.Under the sequence framework,analyzing the statistical of the sedimentary characteristics and building up the facies belt division standard,and the facies belts are classified into I,II and III types.The ? types of facies belt deposited bioclastic bank,the ? types of facies belt deposited melange grain bank,the ? types of facies belt deposited mud flat and interbank sub-sag.Including ?,? facies porosity is greater than 5%,as the favorable facies belt.