Prediction of Stratigraphic and Areal Occurrences of Jilh High Pressure in Bahrain Field

Abstract

ABSTRACT Bahrain field is an asymmetrical anticline trending in the North South direction. The sedimentary column extends from Cambrian Saq Sandstone to the Miocene reefal deposits, exposed on the surface. Jilh Formation is of Middle Triassic age. It is named after Jilh Al-Ishan escarpment, Lat 24° 04′ North, Long. 45° 46′ East, in Saudi Arabia across which the type section was measured. The top of Jilh is marked by a regional unconformity that has caused the complete erosion of the Minjoor Sandstone of upper Triassic and the partial erosion of Jilh Formation in the Bahrain Field. The preserved thickness of the Jilh Formation in Bahrain Field ranges from 800 feet (244 meters) on the crest to 1380 feet (420 meters) on the flanks. Extremely high pressures were encountered in the Jilh Formation at depths ranging from 7000 to 8000 feet (2135-2440 meters). These abnormally pressured zones are usually associated with gaseous brines and have pressure gradients ranging from .65 to .88 PSI/FT (1.5 to 2.03 gram/cc). The lateral pressure compartmentation necessary for creating the abnormally pressured zones is apparently formed by the central graben faults and the perpendicular minor antithetic faults. The pressurized brines are believed to have been sourced from the clayey sections of the Jilh Formation. The Jilh high pressure zones are difficult to drill through unless the depth and the lateral distribution of these abnormal pressures are known. This paper will attempt to describe an approach for predicting the occurrences of abnormal high pressure zones in the Jilh formation ahead of drilling to facilitate the casing design and minimize risks normally associated with such phenomenon.