Geochemical and Isotope Data Aided Remote Sensing Interpretations of Surficial Mineralogical Heterogeneities as Possible Late Diagenetic Indicators of Microseepage in Garza, Texas
Microseepage can cause characteristic surface alterations above hydrocarbon accumulations. Bleaching of red beds, clay alterations, and deposition of secondary minerals are amongst the most commonly recognized hydrocarbon-induced alterations and can be mapped using remote sensing. The spectral properties used for mapping these alterations, however, are not exclusive to hydrocarbons but can also be caused by other reducing fluids. For this reason remote sensing interpretations should be supported by additional geochemical and isotope data for conclusive results. Although recent studies have used such an integrated approach and documented hydrocarbon induced alterations, some of the pioneering studies lacked additional data so as to substantiate the remote sensing observations. Also, the remote sensing analysis of these early studies relied only on user interpretations which inevitably introduced bias. In such a study, discoloration of red beds has been observed from remote sensing data and attributed to hydrocarbon microseepage in Garza County, Texas. Here, we reevaluated surficial mineralogical heterogeneities in the Dockum Group rocks as possible late diagenetic indicators of microseepage in Garza using remote sensing, geochemical, and isotope data. Multispectral satellite data were analyzed using robust image processing algorithms minimizing the user bias. Mineralogical heterogeneities have been identified based on relative abundances of iron oxide and clay minerals in the satellite data. Complementary geochemical and isotope analysis have also been performed on the hand specimens. The geochemical analysis revealed lower iron oxide and higher clay mineral content for the so-called discolored specimens as well as higher weathered feldspar and calcite, which may be attributed to reducing fluids. The remote sensing analysis detected the areas of lower iron oxide and higher clay mineral content. These areas are (i) mostly strata-bound and persistent along the topographic highs and (ii) not restricted within the areas of known hydrocarbon accumulations but have rather large extents which suggests hydrocarbons may not be the only reducing fluid. The isotope results also support the idea of an alternative reducing fluid. Although the discolored specimens are depleted in 13C, it is not as much depletion as expected from hydrocarbons. Based on these observations we conclude that the surface alterations in the area may not be caused by hydrocarbon microseepage.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017