Organic facies in Cretaceous and Jurassic hydrocarbon source rocks, Southern Indus basin, Pakistan

Abstract

A detailed organic petrographic study of the geologic section penetrated by the Sann #1 well in the Southern Indus basin (Kirthar Trough) of Pakistan permits definition of organic facies and oil-generation potential of Cretaceous and Jurassic source rocks. The well encountered Eocene through Jurassic rocks; however, only closely spaced samples of fine-grained rocks from the Cretaceous Goru, Sembar, and the Jurassic Chiltan formations in the well were studied in detail. Kerogen from the samples was examined in blue light epifluorescence and transmitted white light. The petrographic data are compared with total organic carbon and Rock-Eval® pyrolysis data from an earlier study. The results of the present study demonstrate that three distinct organic facies are present within the Sann #1 section. Organic facies A is oil-prone and consists of fluorescent amorphinite with trace to minor amounts of alginite and other liptinites. This facies quite possibly represents deposition in reasonably deep water. In shallower water conditions, a mixture of non-fluorescent and fluorescent amorphinite with some liptinites and vitrinite was deposited. This mixture of kerogen maceral types characterizes organic facies B, which contains both oil- and gas-prone organic material. Organic facies C was deposited under the most oxidizing shallow-water conditions of the three facies. It is largely gas-prone and contains mostly non-fluorescent amorphinite and minor amounts of vitrinitic material. A cyclical variation in kerogen types is characteristic of the studied well section. Such cyclic variation was produced by repeated periods of transgression and regression. If these variations, which produced the different organic facies, are confined to the trailing edge of the Cretaceous Indian subcontinent, then local controls such as the velocity of the subcontinent's northward movement instead of global change in sea level may have determined the extent of water depth along the margin of the subcontinent.