Characterization and Origin of Oxfordian Dolomites in the Western High Atlas, Morocco

Abstract

Fault-controlled dolomitization is known to control reservoir properties in the Cap Guby Field, offshore Morocco, and the Panuke Field, Canada. This study aims to understand the processes that lead to fault-controlled dolomitization, its distribution and origin within the Atlantic margins using outcrops of Oxfordian carbonate sediments in the Agadir-Essaouira Basin (AEB). This post-rift succession overlies syn-rift Triassic and Jurassic sediments, volcanics and salt with localized dolomitization around normal and reverse faults. Syn-rift faults mainly have displacements of about 1-2 km, whilst post-rift faults are commonly salt related with displacements that ranges from 100 m to 300 m.

The Oxfordian succession within the AEB consist of two formations, the lower Tidili Formation, composed of reefal carbonates, and the upper Iggui El Behar Formation, consisting of lagoonal carbonates overlain by marls. Dolomitization is non-stratabound, extending for up to 100 m away from faults with a gradational contact to limestones. The dolostone is often kerogenic and oil stained. Dolostones that occur in vicinity of faults have non-planar textures and are highly fractured and vuggy, although some fractures are cemented by dolomite and/or calcite. The most porous dolostone textures are within packstones and coral boundstones which are dominated by vugs, bimoulds and intercrystalline porosity.

Petrographical analysis reveals five main dolomite and two dedolomite textures. The dolomites are categorized as: 1) microcrystalline; 2) planar-euhedral; 3) planar-subhedral; 4) non-planar, anhedral; and 5) saddle. The microcrystalline and planar dolomites are replacive, whereas, the non-planar and saddle dolomite phases are both replacive and cementing. Dolomite textures vary in time and space. In the Lower Oxfordian: planar, non-planar and saddle dolomite occur, whilst the Upper Oxfordian is dominated by planar dolomite textures. Ongoing field, petrographical and geochemical analysis will now consider the processes governing the formation of different dolostone textures, and hence pore type. Ultimately, this study will deliver a framework that facilitates predicting the presence of dolostone, and its reservoir quality.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018