ABSTRACT: Abnormally High Formation Pressures, Potwar Plateau, Pakistan

LAW, BEN E., U.S. Geological Survey, Denver, CO

Abnormally high formation pressures in the Potwar Plateau of north-central Pakistan are major obstacles to oil and gas exploration and production. Severe drilling problems associated with high pressures have, in some cases, prevented adequate evaluation of reservoirs and significantly increased drilling costs. Two distinct pressure regimes are present in this Himalayan foreland fold- and thrust-belt basin; one in Neogene rocks and another in pre-Neogene rocks. Pore pressures in Neogene rocks reach lithostatic levels and are interpreted as having resulted from tectonic compression and undercompaction associated with high rates of sedimentation. The top of the overpressured section is commonly at depths less than 600 m. Pore pressures in pre-Neogene rocks are generally less than those i Neogene rocks, commonly ranging from 11.3 to 15.8 kPa/m, and most likely were caused by hydrocarbon generation.

The causes of overpressuring in the two pressure regimes are directly related to differences in lithology, depositional environment, and burial and thermal history. Neogene rocks are as thick as 6500 m and are composed of conglomerate, sandstone, siltstone, and shale that were rapidly deposited in fluvial and fluviodeltaic environments in response to collision of the Indian and Eurasian plates. They are organically lean, are thermally immature to marginally mature, and contain relatively fresh water. In contrast, pre-Neogene rocks are as thick as 2500 m and are composed of sandstone, siltstone, shale, carbonate, evaporite, and minor coal that were deposited at lower rates than Neogene rocks in a wide range of marine and nonmarine environments. They are thermally mature, contain modera e amounts of organic matter, and contain relatively more saline water than Neogene rocks. Burial and thermal history reconstructions of these Neogene and pre-Neogene rocks indicate that present-day temperatures are maximum temperatures.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)