Effects of Diagenesis on the Reservoir Quality in the Upper Sands of Lower Goru Formation, Badin Block, Lower Indus Basin, Pakistan

Abstract

The core samples of B member of upper sand reservoir rocks of the lower Goru Formation from three wells of the Badin block were studied using thin section, XRD, and SEM techniques to investigate the diagenetic trends and their effect on reservoir quality. Microscopic study indicates that the B sand unit is mature with quartz as the predominant mineral constituent with variable amount of feldspar and lithic minerals. The QFL plot indicates that most of the samples are plotted in the field of quartz arenite, sub-litharenite and sub-arkose respectively. A few samples belong to the category of litharenite and feldspathic litharenite. The feldspars were partially to completely altered to kaolinite and other clay minerals. Coarse-crystalline or micro-crystalline calcite is the predominant cementing material. Bulk rock XRD analysis also confirms that the main mineral constituents of there samples are quartz and calcite in variable proportions. The undulose extinction and fracturing of quartz grains indicate that the area remained under stress. Moreover, such fracturing is post-depositional and therefore is the product of late diagenesis. Scanning Electron Microscopic (SEM) images at 50 micrometer (μm) size show irregular type of fracturing within the quartz grains. This late stage fracturing of quartz has also generated various types of channels which may serve as secondary porosity. The quartz overgrowth was observed in some samples due to late stage diagenesis. The micro-crystalline cement in the form of calcite is mostly present within the pores in fractured quartz. The results indicate that the diagenesis played a significant role in improving the reservoir characteristics of B sand by increasing the porosity due to quartz fracturing, feldspar dissolution, alteration and quartz corrosion. Hence, better understanding of reservoir heterogeneities in porosity through diagenetic studies can be helpful in evaluating potential reservoir horizons for hydrocarbon accumulation on spatial and temporal scales.