Lacustrine Tight Oil: New Petroleum Exploration Domains in China
Li, Denghua; Yang, Tao
Department of resource evaluation & planning, Research institute of petroleum exploration & development,PetroChina, Beijing, China.
Many tight oil plays have been discovered in China,and the implementation of new technologies,including geochemistry, geomechanics, horizontal drilling, geosteering, and well stimulation,have made it possible to pursue reservoirs that were previously deemed uneconomic.An example of formerly uneconomic plays in China is that of low permeability clastics found in ancient lake beds, where lacustrine deposits involve interbedded and intercalated sand-shale sequences.
Tight oil in China is different from that in North America.Tight oil in China completely comes from lacustrine mudstones and shales,while that in U.S. mostly derives from marine shales.A comparison indicates that lacustrine reservoirs show higher clay content and more heterogeneous.
Lacustrine tight oil plays in China can be divided into three types,including fractured shale plays,such as Cretaceous Qingshankou shale of Songliao Basin,tight sandstones and silts, such as Triassic Yanchang formation of Ordos Basin,and tight carbonates, such as Jurassic Daanzhai formation of Sichuan Basin.Each type consists of lacustrine mudstone or shale source rock with tight interbedded or nearby reservoirs.
There are four basic characteristics of lacustrine tight oil plays.The first one typically has permeability less than 1mD and porosity less than 10%;the second one has high reservoir pressure,0.5-0.8psi/ft,and low oil density,34-70API;the third one is characterized by abundant nanopores in reservoirs;the last one is that high thermal maturity,the vitrinite reflectance 0.9%-1.3%,and high GOR(gas oil ratio) make for high tight oil output.
In the exploitation of lacustrine deposits in China, three horizontal wells have achieved much higher oil production than was previously possible. After a thorough analysis,dolomites in salty lake and siltstones in deep lake,interbedded with mudstones or shales,are more favorable reservoirs,and expanded exploration potential of lacustrine tight oil is forecast in China.
AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012