Master of Science (MS)
Geology and Geophysics
Textural and mineralogical variations within 33 shale samples were analyzed to provide a model for predication of hydrocarbon top-seal efficiency. Variation in mineral abundances in the subsurface influences petrophysical properties and other characteristics of hydrocarbon sealing sequences and reservoirs. In geophysical well logs, various proxies, such as gamma ray emission and neutron absorption, are used to assess changes in mineral composition. This study directly compares X-ray diffraction measurements of mineral percentages and geophysical responses from Middle Miocene cored intervals in the Santa Cruz Well; Mississippi Canyon block 519, Gulf of Mexico with those derived from well logging. Distinct textural differences reveal characteristics that can be used to construct five unique microfacies. Microfacies Interpretation was based on SEM determination of clay/silt fraction ratio, sedimentary structures and XRD mineralogy content. Shale types examined can be divided into 5 microfacies: 1. Finely laminated clayshale; 2. Slightly silty clayshale; 3. Banded mudshale; 4. Slightly sandy mudstone; and 5. Moderately sandy and bedded siltstone. The respective shale microfacies 1 through 5 are a result of varied depositional environments and diagenetic processes produced in specific depositional frameworks and possess unique mineral contents. The depositional environment from each microfacies is as follows: Microfacies 1. Low Energy, Distal Hemipelagite; MF2: Distal turbidite; MF3: Proximal Hemi-Turbiditic (Banded); MF4: Poorly Sorted, High energy, Debrite; MF5: High Energy, Gravity Slump-Mass Transport Complex and Leveed Channel Complex. Accessory mineral constituents discretely identified in individual microfacies provide an indication to seal quality based on inherent subsurface quantities including pyrite, siderite, and carbonate material. Identification of successful sealing parameters (textural, mineralogical, petrophysical) is evident in well logs providing an indication to possible prediction of seal quality based on inferred and actual parallels to previous seal efficiency studies by Dawson and Almon (2002,2006) and Bjǿrlykke et al. (2004, 2008). A successful seal should have high clay-mineral mean abundances, low amounts of silt, high to moderate bulk density and gamma-ray measurements, expandable clays and illite, relative high abundances of pyrite, siderite and carbonate material. Alternatively, a less effective seal typically contains higher amounts of silt comparably, less mean abundance of clay, moderate to low bulk density and gamma ray readings. MF1 and MF2 exhibit the best sealing potential whereas MF4 and MF5 contain less effective textural characteristics of a high quality seal. Qualitative comparisons of summary statistics for distinctive parameters provide visual evidence of microfacies characteristics and variability. Quantitative discrimination of individual microfacies results from varying quantitative mineral contents revealed by multivariate statistics yields a perfect distinction between microfacies end member MF1 and MF5. A random forest classification separated microfacies perfectly and ranked variables according to influence on classification of the viable characteristics necessary to determine predictive properties of successful sealing microfacies from lab derived mineral constituents and geophysical well-log calculations.
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Harrison, Andrew Tyler, "A Petrophysical Evaluation of Factors That Control Cap Rock Seal Quality" (2010). LSU Master's Theses. 2711.