Identifying Facies Control on Diagenesis and Reservoir Quality of an Offshore Mid-jurassic Reservoir

Abstract

A sedimentological study of an Offshore, mid-Jurassic reservoir was carried out in order to understand reservoir quality and reservoir architecture. In-house core description, and analysis of open-hole petrophysical log interpretation and conventional core analysis, enabled the authors to interpret the depositional environment and produce hand drawn facies maps to understand the facies architecture; by developing facies scheme, depending of depositional environment comingled with diagenetic process. About 2,285 ft of core and 1,799 thinsections were described from 18 wells, distributed throughout the field. Description of core was done at 1:50 scale and about 495 thinsections were pictured throughout the reservoir section. Detailed description of textural and faunal content, along with sedimentary features was carried out to determine the depositional model and facies architecture. By analyzing the diagenetic features and porosity type, Reservoir Quality controls were derived. All these elements were taken into consideration in order to build facies scheme that not only justifies the depositional environment but also incorporates the impact of diagenesis. A low relief, very active shoal system on a giant carbonate platform was interpreted. Various Lithofacies were grouped into 5 Facies associations keeping in mind depositional environment and diagenetic impact. This model was used to make 4 hand drawn facies maps representing reservoir X and High permeable layer, X7 with general tendency of better facies in the northern part of the field. The novelty of this work is the differential degree of cementation due to the abundance of echinoderms degrading the reservoir quality, and identifying these dominant echinoderm facies zones allows us better well placement by avoiding them.