Located in West Texas and Southeastern New Mexico, the Greater Permian Basin has recently become the largest petroleum-producing basin in the United States and the second largest in the world, having produced over 30 billion barrels of oil as of January 2018 (Mercador, 2018). Although the Permian Basin has been conventionally drilled since the 1920s, horizontal drilling and hydraulic fracturing have recently generated a resurgence in activity in the once thought uneconomical, low permeability, basinal plays of the Lower Permian (Wolfcampian and Leonardian Series) stratigraphy. The current most popular unconventional targets within the Midland Basin, a sub-basin of the broader Permian Basin region, are the fine-grained, low permeability siliciclastic intervals of the Leonardian Series (Spraberry and Dean formations) and the organic-rich calcareous mudstones of the Wolfcampian interval. The Permian Basin has been subject to a great number of geologic studies to establish age, stratigraphy, regional setting, and depositional facies in support of a long history of conventional oil field reservoir development in the basin. A few recent studies have been conducted on the most popular unconventional play in the Midland Basin, sometimes referred to as the Wolfberry Play (Wolfcamp interval and Spraberry Formation), but fewer yet have studied the Dean Formation. In response to growing industry interest in the Permian Basin, this study focuses on the sedimentology, stratigraphy and reservoir quality of the Dean Formation within Borden and Dawson counties, West Texas. Based on observations, analyses, and interpretations, multiple conclusions were made regarding the Dean formation in this thesis study. The Dean Formation can be divided into two parts, the Upper and Lower Dean, which are separated by a cemented carbonate zone, mineralogically unique from one another, and consequently contain different reservoir properties due to these differences. Overall, the Upper Dean contains a higher percentage of authigenic and detrital carbonate minerals that are prone to occlude porosity and restrict permeability, while the Lower Dean has higher silica content, lower authigenic and detrital carbonate content, and higher overall porosity and permeability. Furthermore, the Lower Dean displays lower water saturation, higher TOC values, and a higher fracture count than the Upper Dean. Six facies were determined through three Dean Formation core descriptions and analyses. All facies can be subsequently broken up into three separate facies associations: Facies 1-3 are basinal facies associations (Facies 1 (Laminated argillaceous siltstone), Facies 2 (Bioturbated argillaceous siltstone), and Facies 3 (Massive/Microburrowed argillaceous siltstone)); Facies 4 and 5 are turbidite facies associations (Facies 4 (Clean siltstone (Bouma sequences Ta, Tb, and Tc)) and Facies 5 (Silty shale (Bouma seqeuences Td and Te)); Facies 6 is a transitional facies association (Facies 6 (Wavy-laminated/rippled stiltstone)). Dean Formation sediments are interpreted as turbidite deposits that have passed through submarine canyons/channels and fans, which were later deposited deep in the basin. Facies within the Dean Formation have both high vertical and lateral variability due to localized turbidite deposits making it difficult to correlate without well logs. Using a technique called core luminance it was found that the standard logging tool resolution used to sample the Dean Formation is inadequate due to the extreme variability of lithology and facies as it typically misses all of these changes. These differences between the core luminance curve and the standard gamma ray curve causes major differences in net sand in the core versus the net sand in well logs. Sediments found in three cores examined exhibited signs of high terrigenous input, moderate amounts of paleoproductivity, and low to moderate amounts of anoxia. XRF and XRD analyses generally showed that silica content within cores increased with depth while carbonate content decreased. Overall, the Dean Formation has relatively low to moderate average porosity values and low average permeability values, ranging from negligible up to ~12% porosity and negligible to 0.4 mD permeability. Facies 1 (average porosity ranging from 9.6 - 9.8% and 0.2 mD), Facies 2 (average porosity ranging from 8.2 - 8.6% and 0.4 mD), and Facies 4 (porosity ranging from 8.7 - 12.0% and 0.2 mD) represented the highest porosity and permeability values while Facies 3 (average porosity of 2.0% and 0.2 mD) and Facies 5 (average porosity ranging from 3.2 – 8.0% and 0.2 mD) had the lowest. Microfractures found within the facies greatly enhanced porosity when present.
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