Title: New Reserves in an Old Field: The “Dakota” (Plainview) Play in the Wattenberg Field, Colorado
Speaker: by John H. Ladd, HS Resources, Inc., Denver, CO
Date: October 19, 2001
Publication: The Outcrop, October 2001, p. 4-5
At Wattenberg Field in the Denver Basin, Colorado, the Plainview Sandstone, commonly referred to by operators as the Dakota Sandstone, lies less than 200 ft below the productive J Sandstone and is within the hydrocarbon-saturated, basin-centered gas accumulation. However, it was largely ignored during the first twenty-five years of active development of the J Sandstone due to drilling difficulties and erratic production results. Since that time, it has been the primary focus of efforts to find new reserves in this mature field, with approximately 60 BCFE developed to date. This talk will address the issues of why the Plainview was ignored for so long and strategies HS Resources has used to economically develop it.
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 flowed 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 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. In general, reservoir quality is more consistent and more predictable in the deltaic and shoreline sandstones than in estuarine or valley-fill sandstones. As a practical matter, the Plainview 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.