Kansas Geological Survey, Open-file Report 91-52, p. 35-55
Phillips Petroleum Company
Campo and Tanner fields, south-central Baca County, Colorado (fig. 1), discovered in 1982 and 1985, respectively, have produced approximately 1 million barrels of oil from a southwest-northeast-trending, faulted anticline. The primary reservoir in the two adjacent fields is a Pennsylvanian (Desmoinesian or Missourian) sandstone. The sandstone and immediately overlying and underlying strata were cored in two wells, one from each field (figs. 2 and 3). The wireline-log character of the cored intervals is shown in figs. 4 and 5.
Figure 1--Location of study area and major Middle and Late Pennsylvanian physiographic and tectonic features of the southern midcontinent.
Figure 2--Core diagram of Cities Service Kearley A-2. [An Acrobat PDF version of this figure is available.]
Figure 3--Core diagram of Cities Service Womack B-1. [An Acrobat PDF version of this figure is available.]
Figure 4--Compensated neutron-lithodensity log of the Cities Service Kearley A-2 showing wireline-log character of core units (core depth = wireline log depth).
Figure 5--Compensated neutron-lithodensity log of the Cities Service Womack B-1 showing wireline-log character of core units (core depth + 2 ft = wireline log depth).
Strata underlying and gradational to the reservoir sandstone are light-colored, well-sorted, calcareous, very fine grained, burrowed (predominately Planolites) sandstone (core unit B) grading downward into gray to dark-gray, fossiliferous, slightly pyritic, argillaceous siltstone and silty shale (core unit A) (plate I A-D). These rocks record a gradual shoaling from an open-marine, offshore, shelf environment (unit A) to a more landward, shallower, relatively low-energy, lower shoreface environment (unit B) (fig. 6).
Figure 6--Paleoenvironmental interpretation for the main Pennsylvanian reservoir sandstone and immediately underlying and overlying strata, Campo and Tanner fields, Baca County, Colorado (Womack B-1 and Kearley A-2 cores).
The reservoir sandstone is approximately 14 ft (4.3 m) thick in each of the two cored wells. The sandstone is predominately planar and low-angle stratified, and less commonly, small-scale cross stratified, fine-grained sandstone and similarly stratified, very coarse grained sandstone and conglomerate (core unit D) (plates I [E-F] and II [A-B]). The lowermost part of the reservoir is a burrowed, fine-grained sandstone (core unit C). The reservoir sandstone reflects a transition from lower shoreface (unit C) through upper shoreface/surf zone to foreshore/beach swash zone (unit D). Mineralogically immature, arkosic sediments composing the sandstones were shed off the emergent Apishapa-Sierra Grande uplift a few tens of kilometers to the west. Core porosities over much of the reservoir sandstone are 10-15%. Porosity types include primary intergranular and minor, secondary, leached feldspar. Primary intergranular porosity is variably occluded by finely to coarsely crystalline, equant calcite cement, authigenic chlorite clay, and minor amounts of quartz and feldspar overgrowths (plate II [C-E]).
The reservoir sandstone is gradationally overlain by low-porosity, variably fossiliferous, poorly sorted, fine- to coarse-grained sandstone with common argillaceous laminations and partings (core unit E) (plate III A). This argillaceous sandstone records upper foreshore and possibly backshore deposition and continued shoaling of the underlying siliciclastic sequence. Gradationally overlying these rocks is a 1-2.5-ft (0.3-0.76-m)-thick interval of argillaceous and sandy limestone, including bioclastic washed packstone, stromatolitic and intraclastic limestone, and nodular limestone with desiccation cracks (core unit F) (plate III [B-F]). These carbonates reflect a decrease in siliciclastic influx and deposition in upper foreshore/backshore to possibly upper shoreface environments (fig. 6).
Dark-gray, silty and calcareous shale and argillaceous siltstone, with fossil assemblage dominated by ostracodes and scolecodonts (core unit G) (plate III G), rest abruptly on the underlying carbonates. These dark, argillaceous rocks indicate a marine deepening and return to quiet-water, offshore, shelf deposition. This deepening may be related to eustatic rise in sea level or the fore-mentioned decrease in siliciclastic influx, ending seaward progradation of the described beach-shoreface sequence.
[Note: An Acrobat PDF file containing all of the plates is available.]
Kansas Geological Survey
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Web version created June 28, 2012. Original publication date 1991.