ABSTRACT: Petrology of the Pennsylvanian Reservoir Sandstone at Campo and Tanner Fields, Baca County, Southeastern Colorado

Craig D. Caldwell

Campo and Tanner fields, discovered in 1982 and 1985, respectively, have cumulated for approximately 1 million bbl of oil from a southwest-northeast-trending, faulted anticline. The primary reservoir in the two adjacent fields is a Pennsylvanian (Desmoinesian or Missourian) sandstone. This sandstone and immediately overlying and underlying strata were cored in two wells, one from each field.

Strata gradationally underlying the reservoir sandstone are light gray and grayish tan, well sorted, very fine-grained, burrowed (predominantly Planolites) sandstone grading downward into gray to dark-gray, locally fossiliferous, slightly pyritic argillaceous siltstone and silty shale. These rocks suggest a gradual shoaling from an open-marine, offshore, shelf environment to a shallower, relatively low-energy, marine environment just seaward of the surf zone (i.e., lower shoreface).

The reservoir sandstone, approximately 14 ft thick in each of the two cored wells, is made up of planar and low-angle stratified, and less commonly small-scale cross-stratified, free-grained sandstone and poorly sorted, similarly stratified, fine- to very coarse-grained sandstone. Coarse-grained layers are interbedded with fine-grained layers in the middle and upper parts of the reservoir. Burrows are locally numerous in the fine-grained sandstone composing the lower part of this interval. The reservoir sandstone reflects a transition from lower shoreface through nearshore surf zone (upper shoreface) to beach swash zone (foreshore). Mineralogically immature, arkosic sediments composing this sandstone were shed off the emergent Sierra Grande uplift a few tens of miles to the west.

Core porosities over much of the reservoir sandstone are 10 to 15%. Porosity types include primary intergranular and minor, secondary, leached feldspar. Primary intergranular porosity is variably occluded by finely to to coarsely crystalline, equant calcite cement, authigenic chlorite clay, compaction, and minor amounts of quartz and feldspar overgrowths.

The reservoir sandstone is gradationally overlain by low porosity, variably fossiliferous, poorly sorted, fine- to coarse-grained sandstone with common argillaceous laminations and partings. This argillaceous sandstone records backshore deposition and continued shoaling of the underlying siliciclastic sequence. Gradationally overlying these rocks is a 1 to 2.5 ft thick interval of argillaceous and sandy limestone, including bioclastic washed packstone, stromatolitic and intraclastic limestone, and nodular limestone with dessication cracks. These carbonates reflect a decrease in siliciclastic influx and deposition in tidal flat to upper shoreface environments.

Dark-gray, silty and calcareous shale and argillaceous siltstone, with scarce macroscopic fossils (mostly brachiopods) and local burrows, rest abruptly on the underlying carbonates. These dark, argillaceous rocks indicate a marine deepening and return to quiet-water, shelf deposition. This deepening may be related to the forementioned decrease in influx of siliciclastics, ending deposition and seaward progradation of the described beach-foreshore sequence.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990