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Description of Intragranular Micropores in Ooid Grainstones with Bimodal Porosity Network from Lower Cretaceous Lekhwair Formation in United Arab Emirates

Abstract

Oolitic reservoirs with bimodal porosity systems produce water-free hydrocarbons despite exhibiting low-resistivity and high-calculated-water saturations from wireline logs. This paradox between resistivity and productivity has been recognized for more than three decades in oolitic reservoirs from Upper Jurassic Smackover. Other examples include Lower Cretaceous Rodessa, Mississippian St. Genevieve, and Cretaceous giant reservoirs from the Middle East. Previous studies postulated that resistivity in oolitic reservoirs is controlled by a connected network of micropores within ooids holding conductive, capillary-bound brine. However, details regarding distribution and connectivity of intragranular micropores are not well-documented. A clear understanding of micropores distribution in oolites will help in the recognition of overlooked pay zones. This study aims to investigate distribution and connectivity of intragranular micropores in low-resistivity oolitic pay units. We present data from two wells producing from Lower Cretaceous Lekhwair Formation in an onshore field in United Arab Emirates. Lekhwair Formation in this part of U.A.E. is characterized by shallow-subtidal Bacinella floatstone, peloidal mud-dominated packstone, and ooid grainstone. Ooid grainstones are considered in detail here since they produced the majority of hydrocarbons. Polished thin sections impregnated with blue-fluorescent dye highlight the distribution of micropores (<10μm). Percentage of microporous allochems was determined using 100 point-counts per slide. Other attributes considered were size, morphology, and crystal contacts of microcrystalline calcite using scanning electron microscopy. On the basis of above-described fluorescent microscopy, we classify intragranular micropores into five types according to their distribution within ooids: outer-rim, inner-rim, nucleus, patchy, and pervasive. Outer-rim and pervasive result in a connected network of micropores between ooids because of frequent interpenetrating grain contacts. Inner-rim and nucleus micropores are isolated from other types of micropores. Patchy micropores show either connected or isolated micropores. Outer-rim micropores represent the predominant type of intragranular micropores and may represent a common style of micropores to low-resistivity oolitic pay units. An impact of this finding could be the discovery of additional pays from oolitic reservoirs which were originally interpreted to be water-bearing.