HYDROCARBON FORMATION IN THE JIALINGJIANG LIMESTONE OF SICHUAN BASIN

Abstract: The gas reservoir in the Lower Triassic Jialingjiang Group is the main gas layer in the Sichuan Basin. There are two different views on its gas source, non-indigenous gas and indigeneous gas. Those who hold the view that the gas is non-indigenous have the following opinions; ( 1 ) The organic carbon content in the Jialingjiang limestone is lower (about 0.1%), but the organic carbon content in the underlying Leping coal series (Upper Permian) is higher (over 0.5%);(2) The organic matter maturity of solid state in the Jialingjiang Group is in the stage between maturity to high-maturity (Ro 0.95 to 1.5%), In addition, the group also produces condensate and light crude oil(about 0.80 in speC₁fic weight).Its δ¹³C value and the heavy hydrocarbon content show an unusual negative corre-lationship that is δ¹³C₁＞δ¹³C₂＞δ¹³C₃.Therefore,the gas of high δsup>131value formed in the Leping coal series and then migrated to the Jialingjiang Group. The author holds the view that limestone-reservoired gas in the Jialingjiang Group is indigenous, and the main reasons are as follows: ( 1 ) The Jialingjiang Group is an interbedding between limestone, and gypsum. The limestone contains organic rocks with rich sapropel, which can be regarded as gas source rocks. This gas-producing layer is closed by the gypsum layer and has no apparent fault connecting with Permian; therefore, the Permian gas could not migrate into the Jialingjiang Group on a large scale and form area gas-producing layer. ( 2 ) The geochemical indices in the codensate and light crude oil of Jialingjiang Group are closely related to the source rocks of sapropel, but they are different from the source rocks of deviated humus in the Leping coal scries. ( 3 ) The maturity of the natural gas in the Jialingjiang Group, whi- ch is determined according to the different values of carbon isotope of gaseous hydrocarbon, is the same as the maturity of solid-state organic matter in the Jialingjiang Group. The maturity of the natural gas in the Indo-Chinese paleobuildup part in Luzhou belongs to high maturity at the early stage. Therefore, δ¹³C₁＜δ¹³C₂＜δ¹³C₃ indicates nomal distribution in carbon isotope of gaseous hydrocarbon, but in the periphery of the Indo-Chinese Paleo-buildup part, the maturity of the natural gas belongs to high maturity at the late stage. And so,δ¹³C₁＞δ¹³C₂＜δ¹³C₃ shows inverse distribution in carbon isotope of gaseous hydrocarbon. It is a reflection of maturity, but not the gas of highδ¹³C₁ value from the Leping coal series, which is mixed into the peripery of the Indo-Chinese paleo-buildup part.