An Integrated Chemostratigraphic and Magnetic Study of the Wolfcamp Formation, Midland Basin, Texas: What Can These Tools Tell Us About Sequence Stratigraphy and Fabric Anisotropy?
The Wolfcamp Formation in the Midland Basin contains high frequency facies variability. The dominant facies present are organic rich siliceous mudrocks, calclithites, and limestones. This study uses chemostratigraphic and magnetic techniques to define a sequence stratigraphic framework and map fabric anisotropy in the Wolfcamp. These approaches identify visually subtle, but potentially significant, facies transitions using semi-quantitative data. Both techniques were performed on four 1 inch plugs per sampling depth to ensure representative data. Chemostratigraphy is capable of highlighting facies shifts within apparently homogenous mudrock successions with greater confidence than is possible through visual inspection. In each sample, data for 23 elements is measured using XRF. These data are clustered into eight distinct chemofacies based on Hierarchical Cluster Analysis using Euclidian distances and Ward's method. XRF data, from Ti, Zr, K, and Al, show a general transgression in the lower 260ft (MD 6700′-6960′) with superimposed higher-frequency transgressions and regressions. This portion of the unit also show consistently high degrees of bottom water restriction, based on Ni and Mo, that begins to rapidly decrease above 6755′. The upper 290ft of the unit shows a regression that is punctuated by alternating zones of carbonate accumulation and clastic accumulation. This upper portion of the examined interval shows episodic alternation between well circulated and restricted bottom water conditions, with restricted conditions becoming more dominant upsection. Anisotropy of magnetic susceptibility (AMS) quantifies the preferred orientation and intensity of magnetic grains in rocks. AMS data show planar, oblate magnetic fabrics in mudrock facies and vertical/sub-vertical, prolate fabrics in carbonate facies. Planar fabrics show enriched K, Al, and Fe content suggesting that these fabrics are controlled by clays minerals. Vertical fabrics preserve high Fe and Ca suggesting that AMS is controlled by ferroan carbonate minerals. Carbonate facies display a low degree of anisotropy (P′) whereas mudrocks show variable P′ values. Variability of P′ in the mudrock facies is likely related overconsolidation (high P′), underconsolidation (low P′) or mineralogical changes. Magnetic susceptibility shows a strong correlation with Fe content, based on Principal Component Analysis. As a result, Fe content could serve as a proxy for magnetic susceptibility and vice versa.
AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016