Mudrock microfabric and porosity : Sarawak and Sabah Basins, Malaysia

Abstract

Mudrock microfabric is complex and heterogeneous and has a significant effect on mudrock porosity and permeability characteristics. This study proposed the need for a simplified representation of microfabric that has widespread application for sedimentary mudrocks of all ages. One of the principal drivers is to develop an understanding of mudrocks and mudrock microfabric that is useful in the exploration for and production of shale gas and shale oil resources. Three different mudrock formations from the Sarawak and Sabah Basins in Borneo, Malaysia: the Oligo-Miocene Setap Shale and Sibuti Formations and the Miocene West Crocker Formation. New sedimentological data presented here have established a marine shelf depositional environment for the Sibuti Formation, an outer shelf to upper slope environment for the Setap Shale Formation, and a deepwater turbidite-dominated environment for the West Crocker Formation. TOC values are generally low (0.2-3.5%) and include both woody terrestrial and marine algal organic material. The relative proportion of ductile minerals (clays and micas) and brittle minerals (quartz, feldspar, carbonate, pyrite) in the sediment inorganic composition are of key importance in the behaviour of the mudrocks during hydraulic stimulation. A new composite microfacies model has been developed that links microstructure, microfabric and microporosity. This is valid for all three study formations and is believed to be more widely applicable to mudrocks in general. Microstructure and microfabric types have a pronounced effect on microporosity style, distribution and network. An important aspect of this model is the recognition that dispersed granular material (primary and diagenetic), bioturbation traces, and disturbed microstructures all disrupt the original microfabric and enhance microporosity. The integration of a multi-scale, multidisciplinary approach has been applied in this study. Although the individual methods are not new, the application of this kind of systematic approach is considered essential for the study of mudrocks and unconventional hydrocarbon potential.