Low Resistivity-Low Contrast Pays Within the Permo-Carboniferous Sandstone Formation of Central Saudi Arabia: Geological Controls and Implications for Missed Pay
The Permo-Carboniferous sandstone reservoirs of Central Saudi Arabia comprise significant accumulations of hydrocarbons. Identifying these reservoirs is a challenge, as the resistivity log sometimes shows low resistivity response (<2 ohm-meters), indicating a high water-saturated zone, or no resistivity contrast between the hydrocarbon-bearing zone and its underlying formation water interval. Understanding the causes of low resistivity and low contrast (LRLC) in these reservoirs requires an integrated approach that includes formation water geochemistry analysis, electrical properties, microporosity, laminations, and a detailed assessment of the petrographic characteristics of the rocks. In this paper, the effect of grainsize distribution, lamination, the presence of clay minerals, cement, and formation water are discussed. These controls were extracted from petrography, scanning electron microscopy (SEM), and X-ray mineral diffraction (XRD) analysis. A regional formation water salinity map was established from geochemical analysis of sampled formation water within several hydrocarbon fields across the area. The analysis results were integrated with wireline logs and suggest that the low resistivity, low contrast phenomenon is controlled by a combination of factors, including: ● The presence of conductive clay minerals, such as illite-smectite and kaolinite mixed layered clays. ● A highly variable grainsize distribution, ranging from very fine to coarse. ● Microporosity within infiltrated clays and within authigenic kaolinite that elevated capillary bound water. ● Bed thickness/laminations (which is often below the vertical resolution of resistivity tools). ● Variation in formation water salinities.
AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018