Far-East Tertiary Coal-Related Petroleum System

LIN, RUI; DZOU, LEON; DOYLE, KEVIN and JARDINE, ERIC, Unocal Corporation, 14141 Southwest Freeway, Sugar Land, TX 77478

Several prolific petroleum provinces in the Far East have been related to Tertiary source rocks deposited in fluvio-deltaic sequences containing coals and carbonaceous shales. These basins contain large quantities of recoverable gas, condensate and volatile oil. This paper, using examples from the Xihu Trough (East China Sea) and the Pattani Trough (Gulf of Thailand), illustrates that coals as well as carbonaceous shales are important sources capable of generating both gaseous and liquid hydrocarbons. An identical suite of biological markers can be correlated from the coal and carbonaceous shale facies containing land-plant debris to the gas condensates and volatile oils. These molecular markers include tricyclic/tetracyclic diterpenoids (isopimarane and phyllocladane) indicative of gymnosperm (Xihu Trough) and bicadinanes (Pattani Trough) indicative of angiosperm. The concentrations of some of these molecular markers can be very high, comparable to those of the normal paraffins. Microscale Sealed Vessel (MSSV) Pyrolysis demonstrates that both the coal and carbonaceous shale facies are capable of generating liquid hydrocarbons (nC₆-nC₃₅) in addition to large quantities of gaseous hydrocarbons. The combined products are indicative of an overall gas condensate or gas volatile oil petroleum system.
Elemental analyses of coals (representing residual sources of hydrocarbon generation) as well as kerogen (maceral) petrography reveal that hydrocarbons (gas and liquid) are continuously generated and expelled from the coal structure, despite the fact that a residual quantity of generated hydrocarbons can be trapped within the coal macromolecular matrix. The main phase of hydrocarbon generation from coals and carbonaceous shales commences at a vitrinite reflectance (VR) of 0.6%; however, significant generation and expulsion of gas condensates and volatile oils occur at 0.8-1.3% VR. The top of gas generation from coals and carbonaceous shales is considered to be at ca. 2.5% VR. Though methane generation proceeds at higher maturities (>2.5% VR), it is volumetrically very small.

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