--> ABSTRACT: New Reserves in an Old Field: the Dakota Play in the Wattenberg Field, Colorado, by J. H. Ladd; #90906(2001)

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J. H. Ladd

H S Resources, Inc

ABSTRACT: New Reserves in an Old Field: the Dakota Play in the Wattenberg Field, Colorado

At Wattenberg Field in the Denver Basin, Colorado, the Plainview Sandstone, commonly referred to by operators in the field as the Dakota Sandstone, lies less than 200 ft below the productive J Sandstone and is within the hydrocarbon-saturated, basin-centered gas accumulation. Despite twenty-five years of active development of the J Sandstone, the Plainview Sandstone was only sporadically tested and developed until 1996, due to drilling difficulties and erratic production results. Since that time, however, it has been the primary focus of efforts to find new reserves in this mature field.

A variety of depositional environments have been assigned to the Plainview Sandstone, which is exposed in outcrops approximately fifteen miles west of the field. These include estuarine channel, tidal flat, coastal marsh, beach and foreshore, barrier bar, and valley-fill. At Wattenberg, the Plainview Sandstone consists predominantly of shoreline and coastal plain sediments that were deposited along the western edge of the Cretaceous Seaway. It can be subdivided into three stratigraphic cycles dominated by sandstone and generally separated by shales. Each cycle contains estuarine or fluvial channel sands that trend generally from the west or north to east or south. These channels end in channel mouth bars or north-northeast trending shoreline sands. Within the boundaries of Wattenberg Field, shoreline sandstones predominate in the upper cycle and channel sandstones predominate in the middle cycle, while the lower cycle contains both. In all cases, there is a general trend, from west to east, from a more fluvial or estuarine-dominated to a more deltaic or shoreline-dominated environment. These rocks have commonly been partially or completely removed due to downcutting along a lowstand surface of erosion and replaced by valley-fill sandstone of the overlying Bear Canyon Member. Bear Canyon channels trend generally east-northeast, a direction parallel to the trend of the main wrench faults that cut across the field.

Although clean, hydrocarbon-saturated sandstone is present in virtually all penetrations and porous sandstone is common, economic production is restricted to only a few sweet spots where sufficient permeability exists. Based on limited core data, the primary control on permeability is grain size, with fracture-induced permeability being of minor importance. Productive sandstones have been discovered and are being developed in all three cycles of the Plainview Sandstone as well as in the Bear Canyon Member. To date, nearly 60 BCFE of gas reserves have been developed.

The areal extent of the basin-centered hydrocarbon accumulation in the Plainview Sandstone is much more restricted than that of the overlying J and Codell sandstones. Unlike the J Sandstone, which is immediately below the major source rocks in the field, the Mowry and Graneros shales, the Plainview lies at least 200 ft below the source beds. The only potential source beds adjacent to the Plainview are thin organic-rich shales in the overlying Skull Creek and locally preserved organic-rich shales deposited between the lowermost sandstone of the Plainview and the underlying Lytle Formation. This physical separation from the major source beds may explain the more limited extent of hydrocarbon saturation. As a practical matter, the Plainview Sandstone is normally only economically developable in two cases: 1) where existing J wells are available for deepening; or 2) where the J Sandstone is being actively developed and the Plainview can be tested in conjunction with the J Sandstone tests.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado