Morrowan Stratigraphy, Depositional Systems, and Hydrocarbon Accumulation, Sorrento Field, Cheyenne County, Colorado: ABSTRACT

Abstract

The Sorrento field, located on the western flank of the present-day Las Animas arch in western Cheyenne County, Colorado, has approximately 29 million bbl of oil and 12 bcf of gas in place in sandstones of the Lower Pennsylvanian Morrow units. The sandstones were deposited in a fluvially dominated deltaic system, and the trap for the hydrocarbon accumulation is formed by pinch-out of this deltaic system onto regional dip. The primary reservoirs are point-bar deposits. At the Sorrento field, the basal Keyes limestone member of the Morrow formation rests unconformably on the Mississippian St. Louis Formation. Above the Keyes limestone, the Morrow shale is 180 to 214 ft (55 to 65 m) thick, and locally contains reservoir sands. The Morrow shale consists, in ascending order, of: (1) a lower marine shale averaging 40 ft (12 m) thick with minor limestone, siltstone, and sandstone; (2) a deltaic regressive sequence 10 to 65 ft (3 to 20 m) thick consisting of shoreline siltstone that grades laterally into channel-mouth siltstone and sandstone, flood-plain mudstone and coal, fluvial sandstone and conglomerate, levee deposits, and abandoned-channel mudstone; and (3) an upper marine shale averaging 105 ft (32 m) thick with minor limestone and siltstone. The deltaic system prograded from northwest to southeast into a shallow, low-energy sea. The delta was inundated subsequently by regional transgression. The fluvial system of the delta was confined by levees to a meander belt; within this belt, the streams maintained a meandering character to the channel mouth. The major reservoir facies consists of fining-upward grain-size sequences of conglomerate and sandstone up to 55 ft (17 m) thick which are interpreted as point-bar deposits. Individual point bars within the field are characterized by sharp bases, lobate geometry formed by thinning toward the margins due to loss of section from the top, and diameters of 5,200 to 6,500 ft (1,600 to 2,000 m). The bases of the bars consist of very coarse sandstone and granular conglomerate with rip up clasts of shale and coal. Where complete sequences are developed, the bars fine upward to fine-grained sandstone interbedded with shale at the tops. The point bars are overlain by marine shale with little reworking of the upper parts of the bars by marine energy. Channel-mouth bar deposits are developed only locally and are generally silty and tight. One well has encountered reservoir-quality channel-mouth bar sandstone which is distinguished from point-bar sandstone by better sorting, stratigraphic position, and finer grain size lacking the basal, very coarse sandstone and conglomerate. Gas/oil and oil/water contacts are not uniform through the field owing to discontinuities between separate point bars. One such discontinuity is formed by an apparent mud plug of an abandoned channel separating two point bars on the southeastern end of the field. In a well 7,000 ft (2,100 m) from the edge of the meander belt, the regressive sequence is represented by a shoreline siltstone unit 8 ft (2 m) thick with flaser bedding, graded bedding, load structures, and rare wave-ripple cross-bedding overlain by 3 ft (1 m) of flood-plain mudstone and coal with no indication of proximity to a nearby sand system. End_of_Article - Last_Page 1326------------