Development Characteristics Of Lower Carboniferous Carbonate Rocks And Their Effects On Natural Gas Accumulation In Marsel Area,South Kazakhstan

Marsel area in Southern Kazakhstan is located in the Silk Road Economic Belt.The Lower Carboniferous carbonate rocks in this area is rich in natural gas;however,the characteristics of source rocks and distribution pattern of carbonate reservoir rocks are still unclear,which hinders the further development of hydrocarbon gas exploration.In this paper,after analyzing geochemical and petrophysical experimental results and data from logging,core observation,sedimentation and tectonics,we firstly study the source rock characteristics,especially their sedimentary environment,organic matter source,hydrocarbon generation potential and history;then we figure out the prominent reservoir space and conclude microscopic pore structures characteristics of various carbonate reservoir spaces;the controlling factors for natural gas distribution were subsequently discussed.The results demonstrate that the kerogen in Marsel area is dominated by Type III and generally in the stage of mature to highly mature.Characterized by most organic material originating from higher terrigenous palnts and a few aquatic organisms,the organic matter from Lower Carboniferous carbonate source rocks were deposited in strongly reducing to weakly oxidizing environment.Controlled by deep burial and high heat flow in Permian,source rocks have entered highly mature stage during early Permian,and the generation and accumulation of hydrocarbon were subsequantly stopped because of the tectonic uplift.The Lower Carboniferous carbonate rocks can be divided into three types: dissolved reservoir with microporous pore throat(type I),fractured reservoir with submicron-nanometer pore throat(type II)and tight reservoir with nanometer pore throat(type III).The contents of different minerals have significant influence on the heterogeneity of different pore structures,namely the higher the clay content,the stronger the nanopores hererogeneity;the higher the quartz contents,the stronger the micronsubmicron pores;and heterogeneities of different pores increase with the rise of dolomite content.Based on comprehensive analyses,natural gas is enriched in areas which are characterizing by active faults,high hydrocarbon generation intensity,developed reef-beach face and strong dolomization.The research results can guide the tight gas exploration in Marsel area.