Sealing Capacity of Salt and Rubble Zone: Signs From LWD and Mud Gas

Abstract

Salt deposits in sedimentary basins have long been considered to be a seal against fluid penetration and hydrocarbon accumulations are often associated with them. The extremely low permeability of salt is a consequence of the large salt-brine dihedral angle that prevents percolation at low porosities. At the higher pressures and temperatures, however, the salt-brine θ decreases below 60° and allows the formation of a percolating pore network (Lewis and Holness, 1996). This network may act as flow conduits even at very low porosities, affecting underground brine/water migration path and accumulation of hydrocarbons below salt. This offers an elegant explanation for field observations of oil impregnated salt (Schoenherr et al., 2007). The aim of this study is to investigate the sealing integrity of the rock salt and effects of the rubble zone below the salt by studying LWD logs, drilling logs, mud gas data and sample description in mud logs. In this work, 48 wells from 14 subsalt prospects in deep water Gulf of Mexico have been chosen as case studies. Gamma ray, deep resistivity, gas chromatography data (C1–C5) and mud log sample descriptions are plotted and compared with each other to evaluate the sealing capacity. From 48 wells considered in this study, 57% of them agree with Lewis and Holness (1996) criterion for permeability in salt, 14% disagree and the rest cannot be evaluated based on available data. At least two complicating factors affect the validity of Lewis and Holness criterion: rubble zone thickness and depth from base of salt (BOS) to first hydrocarbon accumulation. Analyzing the amount of hydrocarbons embedded in salt as a function of these two factors shows that the rubble zone adds to the sealing capacity at BOS. This study also shows that the vertical extent of hydrocarbons in salt, indicated by gas, fluorescence, oil staining, dead oil and oil cut in salt, is positively correlated with commerciality of discoveries.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015