The Effects of Mineral Fractionation by Turbidity Currents on Reservoir Quality: A Study From the Aspen Field, Northern Gulf of Mexico

Abstract

Turbidites are prolific hydrocarbon reservoirs, yet numerous studies document that reservoir quality (porosity and permeability) prediction is a difficult challenge. In this study, we examine core from the Aspen Field, northern Gulf of Mexico to test how sedimentation processes by turbidity currents impacts reservoir quality in turbidite reservoirs. Strata in Aspen Field consist of a series of interbedded, Upper Miocene sandstone and mudstone intervals that are interpreted to reflect deposition in a distributive channel-lobe system. Five facies are identified, which differ by interpreted sediment-transport mechanism and interpreted flow type. The vertical succession of facies in core is interpreted to record the compensational stacking of 5 lobe elements, resulting in the vertical juxtaposition of the axes, off-axes, and margins of the lobes. X-ray diffraction analysis documents axis-to-margin variations in the composition of turbidite lobes, particularly with regard to the amount of volcanic glass and its diagenetic byproduct, clinoptilolite, resulting in abrupt vertical variations in porosity and permeability. Clinoptilolite, which results from the devitrification of volcanic glass, is enriched in finer-grained, argillaceous facies, resulting in increased cementation and reduced porosity and permeability. The argillaceous facies are interpreted to be from the lateral and distal margins of turbidite lobes. We propose that because volcanic glass is highly angular and has a relatively low density relative to the other mineral grains in the Aspen Field, turbidity currents preferentially fractionate the glass grains to the lateral and distal margins of lobes, where they alter to clinoptilolite during diagenesis. Understanding how fractionation of mineral grains relates to spatial changes in porosity and permeability has implications for predicting vertical changes in reservoir quality in a variety of turbidite-hosted reservoirs. Results of this study can be used to infer similar patterns in other turbidite systems that contain a diverse mineral assemblage with variable settling velocities.

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