Facies Linked to Petrophysical Rock Types in a Permo-Triassic Carbonate Gas Reservoir to Improve Volumetrics and Reservoir Dynamics Prediction

Abstract

A major Permo-Triassic carbonate gas reservoir that was deposited on a very broad, shallow, restricted marine platform across the Arabian plate consists of interbedded carbonates and evaporites with episodes of minor windblown clastic influx. The reservoir has several characterization challenges including: heterogeneous mineralogy, rapidly varying sediment layers, constrained grain sizes (indicating very low initial energy differentiation in the sediments constituting the facies) combined with subsequent lateral reworking in the transgressive systems tract, thin parasequences, aerial exposure, lateral reworking and multiple episodes of diagenesis. During more than three decades of production, many studies have attempted to characterize this formation for the optimization of the gas production. Matching the production history and predicting the dynamic behavior of current and planned wells in this reservoir is still a difficult task. Highly variable mineralogy and pore types suggest significant vertical and lateral variations in the reservoir property parameters used to determine reservoir gas saturation and productivity. This work focuses on the integration of the detailed depositional facies model with the Pressure Depletion Petrophysical Rock Types (PDPRT) developed by Clerke and Al-Nasser to improve the reservoir performance prediction. We use a comprehensive (~1000 feet of core covering ~220 depositional para sequences) set of cored wells and a carefully designed core analysis program to develop a database defining important links between facies and PDPRT's. Of the nine depositional facies defined by sedimentologists, five of them have reservoir potential. The results from this thorough program improves the hydrocarbon saturation calculation and the prediction of reservoir dynamics during pressure depletion. This state of the art characterization workflow includes: core description, thin section examination, petrographic analysis, mineralogy at multiple scales, routine core analysis (RCA) at multiple overburdens, mercury injection capillary pressure (MICP) measurements, porous plate data, and Archie parameter determination. The Pressure Depletion Petrophysical Rock Types (PDPRT)-pore types for the highly variable carbonate lithology are defined using a two stage classification: first on the continuous mineral framework defined from QEMSCAN (Quantitative Evaluation of Minerals by Scanning Electron Microscopy) mineralogy images and then by the dominant pore type using quantitative petrographic data. These PDPRT's-pore types are also completely characterized by their Thomeer pore system parameters obtained from analyzed MICP data. These data define the pore throats of the rock-pore types in detail and with greater petrophysical rock type contrast than the conventional poro-perm method. We obtain and also present here the significant links discovered between the depositional facies and our petrophysical rock-pore types. Integrating depositional (and depositionally related diagenetic) patterns with petrophysical rock typing greatly improves the reservoir dynamics prediction. Additional improvements come from the observation that early anhydrite reservoir pore cements result from the vertical juxtaposition of cycle-capping, tidal-flat facies with reservoir bodies in underlying parasequences. These links significantly improve reservoir model water saturation calculations and permeability predictions, which then leads to improved well placement, reduced CAPEX, production optimization and improved OGIP estimates.