Fluid and lithology discrimination using integrated facies, petrophysical modelling and (AI and vp/vs) rock physics template analysis derived from well data in the carbonate fault-controlled reservoir: A case study from the North Beda Field, Amin High, Sirt Basin, Libya

Abstract

The rock physics template (RPT) represents a trustful method to understand the reservoir properties and attempts to predict the reservoir’s behaviour under various porosity levels, water saturation and clay content. However, many RPT applications were applied on clastic rocks compared to carbonates due to their complex pore systems. Therefore, this study attempts to construct a suitable RPT for the Beda carbonate reservoir, with 3D facies and petrophysical models based on well log data collected from 41 wells of North Beda Field in the western Sirt Basin. Seven wells include core permeability, porosity and lithology data. Two main bounding faults are identified based on well-correlation and divided the reservoir architectural structure into an exposed fault-block and also have controlled subsurface reservoir heterogeneities, lithofacies and petrophysical parameters distributions. However, core data analysis has revealed that the Beda reservoir is made up of four main types of lithofacies, which are classified as Lithofacies A (limestone), Lithofacies B (dolostone), Lithofacies C (shale) and Lithofacies D (shaly limestone) deposited in a wide range of shallow-marine environments. Lithofacies A and B are categorised as having good to excellent carbonate reservoir quality with an average 23% porosity and > 160 mD permeability. According to the log response characteristics, the reservoir consists of a series of electrofacies patterns, which are divided into nine isolated vertical units, reflecting differences in petrophysical properties. The AI-v_p/v_s crossplots show that the shale and clean carbonate line can easily be distinguished. A higher shale content zone is characterised by increased v_p/v_s ratios and decreased AI. Additionally, crossplots with porosity colour-scale successfully divided the reservoir into three porosity zones: excellent, fair to very good and poor. Crossplots comparison between two wells from hanging and footwall sides shows that the shale volume and the water saturation have increased in the hanging wall well and decreased in the footwall well, which indicates an increasing in the hydrocarbon saturation with a relative decrease in acoustic impedance and v_p/v_s values. Results prove that faults and fractures played pivotal roles in enhancing reservoir quality in the high area by enabling larger volumes of fluids to be filtrated and migrated. Besides, the RPT provides robust control of well log data and elastic properties to differentiate the lithology and fluid content in the carbonate reservoir.