Sedimentologic studies of Upper sands of Lower Goru Formation based on well cuttings and wireline logs from wells of X Field in the subsurface of Sindh Monocline, Southern Indus Basin, Pakistan.

Abstract

Detailed studies of lithologic successions of upper sands of Lower Goru Formation in the subsurface of X Field of Sindh monocline were investigated by Zoom Stereo Binocular examination and thin section study of well cuttings in conjunction with the wireline log and core analysis to understand the depositional environment. The sedimentological studies of the well cuttings samples of each of the main bodies resolved into various lithofacies based on their textural signatures (Grain size, shape, and sorting) wireline log and core analysis. Within A, B and C-sand distinct coarsening and fining upward sequences have been identified which reflect frequent sea level fluctuations. Textural investigations, particularly of those samples composed predominantly of sand size fractions, show that the mean grain size in studied field is fine to medium grained and range between 1.8φ and 2.5φ. The samples are moderate to well sorted and well rounded to sub-angular in shape. Mineralogically, the samples comprise mostly of quartz grains and occasionally feldspar grain. Only the fine grained sandstones of studied field exhibit a slight increase in the amount of feldspar. A few grains of dark colored minerals can occasionally be seen in thin sections. The results further indicate that the sandstones are fairly mature. The strongly coarse skewness determined in some of the facies, in particular the lithofacies of B sand indicate the progressively increasing competence of flood flow at near shore areas. Based on overall results it can be interpreted that the Upper sands of Lower Goru Formation of Lower Cretaceous age in Sindh monocline were deposited in moderate to high energy near shore sedimentary environment largely influenced by frequently fluctuating sea level.