Organic Facies in Cretaceous Hydrocarbon Source Rocks, Indus Basin, Southern Pakistan

ROBISON, C. R., Texaco E & P Technology Dept., Houston, TX

A detailed organic petrography study of the section penetrated by the Sann #1 well in the southern Indus basin of Pakistan permits further definition of organic facies and oil-generation potential of Cretaceous source rocks. Closely spaced samples from the well were studied by epifluorescence and transmitted white light. The petrographic data are augmented by pyrolysis data from an earlier study and the petrographic sample suite matches that of the earlier pyrolysis work. Results of the study demonstrate that three distinct organic facies are present within the Sann #1 section. Organic facies A is oil-prone and was found to consist of fluorescent amorphinite with trace to minor amounts of alginite and other exinites. This facies quite possibly represents deposition in reasonably deep water. In shallower water conditions, a mixture of non-fluorescent and fluorescent amorphinite with some exinites and vitrinite was deposited and characterizes organic facies B, which is both oil and gas prone. Organic facies C was deposited under the most oxidizing shallow water conditions of the three facies. This facies is largely gas-prone and was found to contain mostly non-fluorescent amorphinite with some minor amount of vitrinitic material. A cyclical variation in kerogen types is characteristic of the studied section. Such a cyclic variation was produced by repeated periods of transgression and regression.

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