Study on Constitutes of Gas-bearing Measures and Their Depositional Environment

Wang Qi; Shi Jian; Wen Changqing;Ma Baoling(Lanzhou Institute of Geology, Chinese Acedemy of Sciences, 730000) Gas- bearing measure, defined as a complete series of rock assemblage which is favorable for the gas generation, is the geological base of discussing gas formation.It is mainly composed of source rock, reservior and cap rock.Source rock of gas can be generally divided into three types in terms of lithology: Pelite,carbonate and coal rock.According to pete types, gas reservior can be divided into primary POre-type, dissolution pore-type and fissure-type reservior.Generally, conditions of cap rock required for gas accumulation are extremely strict.Considering lithology, cap rOCk can be also divided into three types, that is, polite, gypsum rockand tight carbonate rock.Because of lithologic differences, gas-bearing measures can be classified into four assemblagetypes, namely, Pelite-Sandstone-Pelite, Pelite-limestone-Pelite, limestone-limestone-Pelite and limestone-limestone-gypsum.The assemblage type and distribution characteristics of gas-bearing measures are changed along with their geosraphic location and chronology.Formation of gas-bearing measure has a close relation to the depositional enviroment,ofwhich the most fayorabale one is generally thane sites with shallow water body, including bothmarine and continental factes tract.Sedimentary factes in favor of formation of gas-bearingmeasure in marine tract are as follows:1) coastal marsh coal-bearing elastic rock factes;2) tidal platform carbonate rock factes;dibsrrier platform carbonate-gypsum rock factes;②.confined platform carbonate rock factes;3) deep and shallow water open platform carbonate factesIn continental basins favorable enviroments include:1) semi-deep lacustrine to deep lacustrine elastic rock factes;2) shallow-lacustrine elastic rock factes;3) fluvial and lacustrine delta elastic rock fscies;4) fluvial and lake shore elastic rock factes.The most favorable depositional environments forming source, reservior and cap rock of gasbearing.measures are coastal marsh to shallow plate form in marine basin and the shallow lacustrinefactes in continental basin.In other words, formation of gas-bearing measure does not need deepwater environment, and shallow semi-reduced one is enough.Usually, gas-bearing measures arecompounds of different factes which overlapped and repeated to form complex rock assemblage andregular sequences.This is particularly necessary to the formation of gas source, reservior and caprock assemblage.The valuable formation procedure of three basic elements mentioned above is,firstly the formation of reservior,secondly cap rock and finally the evolution of organic matter insource rock.From the reseach of several cases, the procedure is considered as the most importantand decisive factor to the formation of gas field.