Year of Award

2021

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Department or School/College

Geosciences

Committee Chair

Marc S. Hendrix

Committee Co-chair

Michael H. Hofmann

Commitee Members

Kent D. Sugden, Anna S. Phelps

Keywords

diagenesis, Midland basin, Wolfcamp Formation, reservoir quality, Permian

Publisher

University of Montana

Subject Categories

Geology | Sedimentology | Stratigraphy

Abstract

This study focuses on documenting the diagenesis and pore evolution within the lower Permian Wolfcamp B Formation in two producing wells from the Midland Basin of west Texas. Both wells are located near the northeastern margin of the Midland basin, with well A located proximal to the basin margin and well B located about 20km down depositional dip. The entire Wolfcamp B interval has been cored from both wells.

In both wells, the Wolfcamp B consists of calcareous and siliceous mudstone, skeletal wackestone, packstone, grainstone, rudstone, and floatstone facies that were deposited below wave base by a combination of suspension settle-out and gravity flow mechanisms. These facies are unevenly distributed between the two wells with the proximal core from well A containing an abundance of coarse carbonate beds with subordinate mudstone facies and the distal core from well B containing a dominance of mudstone facies with fewer beds of coarse carbonate.

Using a systematic combination of visual core analysis, petrography, XRD, and SEM-EDS results from this research reveal that the two wells have differences in the major porosity forming and reduction diagenetic events. In well-sorted coarse carbonate facies from the proximal well A, early dissolution of low-Mg calcite and aragonite bearing skeletal grains created abundant macroscopic secondary porosity. Subsequent lining of pores by calcite cement decreased the effects of physical compaction during additional burial, facilitating the preservation of moldic porosity. A later stage of anhedral to subhedral nearly stoichiometric dolomite significantly reduced, but did not completely occlude, the moldic porosity. In contrast, coarse carbonate facies in the distal well B do not exhibit macroscopic moldic pores comparable to those in well A. This variation in the occurrence of moldic porosity between wells is interpreted to be related to variations in the abundance of different skeletal grains among coarse carbonates between the locations. In well A, coarse carbonate beds exhibiting moldic porosity are well-sorted and dominated by heavily micritized foraminifera and grapestone grains that were selectively dissolved, whereas in well B coarse carbonates are dominated by skeletal fragments from other benthos that were not selectively dissolved. Instead, coarse carbonate facies in well B contain no moldic porosity and display replacive zoned magnesian dolomite rhombs with ferroan rims encased in pore filling calcite cement. Dolomite often exhibit intracrystalline micropores and contain numerous illite, feldspar, and pyrite inclusions. Findings from this study provide a better understanding of down-dip variations in pore evolution within the Wolfcamp B, thereby improving the predictability of reservoir quality.

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