Overpressures Induced by Compaction Disequilibrium Within Structural Compartments of Murree Formation, Eastern Potwar, Pakistan

Abstract

Overpressure conditions in Potwar Basin of Pakistan pose significant drilling and completion problems. Specifically, in eastern Potwar, abnormally high pressures are observed within structural compartments of Murree Formation of Neogene age. Compaction disequilibrium and tectonic stresses are considered as two major factors, controlling pore pressures in this foreland basin. Undercompaction of sediments due to high sediment influx affects the porosity-depth relationship as the rock deviates from the normal compaction trend. Demarcation of structural geometries is critical in understanding abnormal pressure conditions within thick molasse deposits of Murree Formation. Eastern Potwar is divided into three zones on the basis of structural geometries, including duplex thrust sheets, salt-cored anticlines, and triangular zones. Geohistory analysis and pore pressure prediction have been performed for determining the overpressuring mechanism. Log-based pressure prediction in different oil fields of eastern Potwar depicts anomalous pressure conditions in some of the wells. High-sedimentation rate during Miocene times resulted in a low porosity reduction rate due to the inability of fluid to escape out of the pores. Eaton’s method was used to predict the pore pressure, and these pressure curves were calibrated against measured pressure to validate the results. Pressures predicted in Qazian-1X well showed very good correlation with measured pressure data. Most of the wells with overpressure intervals exhibited high porosity values, showing deviation from the normal compaction trend. This pressure prediction revealed that compaction disequilibrium is the primary cause of overpressure conditions within Murree Formation, resulted in response to the rapid rate of sedimentation. The deformation mechanism and presence of different structural geometries also contributed in the development of abnormal pressures.

Carbonate Reservoir Quality Variations in Basins with a Variable Sediment Influx: A Case Study from the Balkassar Oil Field, Potwar, Pakistan

“…54 If the sedimentation rate is slow, rocks are compacted in the normal compaction trend, with maintaining the equilibrium between the increasing overburden and fluid expulsion. 53 Sakesar is a proven reservoir in the Potwar Basin, 44 but the formation is mostly water-saturated and has poor reservoir capability in terms of fluid accommodation. Porosity is one of the most important factors that determine whether or not a petroleum accumulation may be economically viable.…”

“…This suggests that the overburden pressure increased, resulting in porosity reduction and fluid expulsion, as the deposition continues . If the sedimentation rate is slow, rocks are compacted in the normal compaction trend, with maintaining the equilibrium between the increasing overburden and fluid expulsion …”

“…Both events are marked by the Indian and Eurasian post-collisional depositional changes during the Miocene times. Uplifting of the Indian plate was followed by the high sedimentation influx of the molasse sediment, which resulted in an increase of lithostatic pressure . A high subsidence rate in response of the increased sediment influx could possibly cause the overpressuring in the Eocene carbonates.…”

“…A line group is manually drawn on the GR log curve, which is taken to be around 70 API to separate the clay points from the non-clay ones. Overburden pressure or the lithostatic pressure is the major reason for the fluid expulsion form the pores. , Overburden pressure (OB) can be calculated using the density data. To predict the pore pressures, OB is required, which is a crucial step in this process since the pore pressure and fracture pressure are often calculated directly from the OB.…”

“…Geohistory analysis is used for determining the burial history (subsidence and uplift) and its related processes controlling the compaction of sediments . Burial history represents all the changes with respect to the compaction and heat flow.…”