The Comparative Research On The Characteristics Of China Typical Marine And Continental Source Rocks Of Shale Gas

The source rocks of shale gas are widespread in China.It is believed that the shales in China are featured by various types and complex depositional environment.Considering the depositional environment,there are three types of source rocks of shale gas developed in China,including marine facies,lacustrine facies and marine-continental transitional facies.There are significant differences of geological features between the three types of source rocks.The accumulation conditions and development prospects of the shale gas can be affected seriously.The Lower Cambrian Niutitang Formation and Lower Silurian Longmaxi Formation of upper Yangtze region can be an example to take a research on the marine shales of China.The shallow shelf and deep shelf are the main forming environment of these shales.The remarkable features of Niutitang Formation and Longmaxi Formation are high abundance of total organic carbon,high layer thickness and high abundance of brittle minerals.There is a positive relationship between total organic cabon and brittle minerals of the marine shales.For the negative influence,the thermal evolution degree of the organic matter is on the over-mature stage.The gas-generated potential of marine shale is very low or depleted.The reservoir space of marine shale is reducing because of a decreasing trend of the organic matter porosity,the main pore of marine shale.In addition,unlike American shale gas,the preservation condition is very important to the south marine shale of China.Overall,the regional structural background,the top and bottom barriers and the trap condition of the shale have a great influence on the marine shale of China.There are significant differences between the marine and continental shale of China.The Chang 7 member of Yanchang Formation in Ordos Basin can be an example to take a research on the lacustrine shales of China.Semi-deep and deep lacustrine facies are the main depositional environment of the Chang 7 member of Yanchang Formation in Ordos Basin.High abundance of total organic carbon and high layer thickness are the remarkable features of the shales in the central lake.The thermal evolution degree of the organic matter is on the mature and high-mature stage.Gaseous hydrocarbon generation can be found in Chang 7 member shale in part of the region of Ordos Basin.The abundance of brittle minerals is low in the lacustrine shale because of the influence of the terrigenous clastic.There is a positive relationship between total organic cabon and clay minerals of the lacustrine shales.In the exploration and development of shale gas,the total organic carbon content and the brittleness of the shale must be taken into account.Unlike marine shales,the porosity of lacustrine shales is low because of low quantity of the organic matter pore.Main of the shale gas is adsorption gas.In addition,main source rocks of the continental basins in China have a high degree of depth,partly deeper than 5000 meters.It will be harmful to increase the cost of the development of shale gas.The shales of the Ordos Basin and Sichuan Basin are more suitable for development because of the uplift after subsidence,with the depth of 2000-3000 meters now.