الملخص الإنجليزي
Core and log data from Lower Cretaceous limestones of the Upper Shu'alba Formation in an oilfield of northwestern Oman were examined with focus on the "argillaceous zones," intervals of widely varying gamma ray (GR) activity regarded as the basal-transgressive portions of a series of low-angle clinoforms. Data from 15 vertical wells, including four with core, and four horizontal well cores were projected into a dip-oriented cross-section derived from a static reservoir model as a basis for visualizing the distribution of clay, lithofacies, and porosity in the two clinoforms represented in the studied oilfield.
The upper (younger) of these two clinoforms is thinner than the lower clinoform (27 vs. 36 meters maximum thickness on average), perhaps indicating a less pronounced transgressive/regressive cycle. Except within this proximal extent of the upper clinoform, the argillaceous zones have total porosity mostly <10% and baseline-normalized GR >23 API units, reflecting clay contents of around 10-18%. In contrast, the reservoir zones have lower GR and porosities of 10-35%. Higher clay content may be linked with lower porosity through facilitation of both mechanical and chemical compaction, with stylolitic dissolution providing a local supply of calcite cement. XRD analyses show that the clays are kaolin, illite/smectite, and illite, similar to overlying Nahr Umr shale.
Lithofacies in the argillaceous zones vary in a proximal direction from mudstone to wackestone to mud-dominated packstone, whereas the reservoir zones extend this range to include floatstone, rudstone, and boundstone. Lithofacies are arranged in a pattern of decreasing water depth and increasing energy both upwards and in the landward direction within each dinoform.
