Characterization of Pore Geometry and Natural Fractures, Little Sand Draw Field, Big Horn Basin, Wyoming: ABSTRACT

Abstract

Little Sand Draw is an asymmetric anticlinal reservoir located on the southwest margin of the Big Horn basin. The field covers 479 acres and has 26 producing wells. Most of the field's production is from Ervay dolomites in the Upper Phosphoria Formation (Permian). The field produces high water-cut oil and has undergone rapid pressure decline throughout its production history. A geologic study was undertaken to determine pore geometry and the abundance of natural fractures. The Ervay dolomite, which has been cored in 14 wells, can be subdivided into six zones. From bottom to top, these are (1) basal phosphatic packstone and siliceous nodular mudstone, (2) phosphatic skeletal wackestone, (3) burrowed silicified mudstone, (4) coarse skeletal packstone, (5) peloid, ostracod packstone to grainstone, and (6) a thin (2-4 ft thick) capping anhydrite. This zonation is remarkably consistent throughout the field. Moldic and microintercrystalline porosity predominates in the reservoir. Moldic pores have significant hydrocarbon storage capacity but are not interconnected. Permeability of the microcrystalline dolomite matrix is low (< 10 md) and cannot account for observed fluid production rates. Open fractures are present in all cores and contribute significantly to reservoir drainage. Fracture intensity is controlled by lithology and structural position. Silicifiedmore » horizons are typically more highly fractured than other zones. Wells with the greatest fracture intensity are aligned along the steeply dipping west flank of the field. East-northeast fracture orientations identified on Schlumberger Formation Micro-Scanner logs agree with directional flow results from pressure interference tests.« less