Reports: DNI849950-DNI8: Heavy Mineral Sorting Processes and their Geochemical Detection: Development of a New Interpretive and Stratigraphic Tool in the Upper Permian Brushy Canyon Formation, West Texas

Michael Tice, Tice , Texas A&M University

The objective of this project is to identify and characterize depositional and diagenetic processes producing siltstones and sandstones of the Middle Permian Brushy Canyon Formation. To date, we have conducted three field trips to measure sections and collected samples in outcrops of the Salt Flat Bench. We have measured elemental spatial distributions, organic biomarker abundances, and carbonate isotopic compositions in outcrop and core samples. This work has resulted in two M.S. theses, three papers submitted for publication, and it is contributing to the thesis research of two Ph.D. students. Three major lines of investigation have emerged from our initial results.

Windblown Dust

Windblown dust is a potentially important but difficult-to-quantify source of siliciclastics for sedimentary basins worldwide. Identifying windblown deposits requires distinguishing them from other low density suspension transport deposits. For instance, laminated very fine grained sandstones and siltstones of the Brushy Canyon Formation have been variously interpreted as 1) the deposits of slow-moving, low-density turbidity currents, 2) distal overbank deposits of turbidity currents, 3) the deposits of turbulent suspensions transported across a pycnocline (interflows), and 4) windblown dust. This facies forms the bulk of Brushy Canyon Formation slope deposits, so understanding its origin is critical to understanding the evolution of the basin as a whole.

Our geochemical mapping shows that these rocks are up to two times enriched in very fine sand sized zircon grains relative to Bouma A divisions of associated turbidites with similar grain sizes, suggesting substantial turbulence during transport that prevented grain interaction with bed surfaces. However, the laminated sandstones and siltstones never show evidence of scour or amalgamation, implying that flow turbulence did not interact with underlying beds. Organic biomarkers preserved in ramp sandstones and siltstones show ten times more variability in terrestrial organic content than channel-filling turbiditic sandstones and siltstones, suggesting that background slope deposits are variable mixtures of windblown terrestrial material and marine pelagic material. Collectively, these observations are most consistent with windblown interpretations for Brushy Canyon Formation slope sediments. If correct, this implies that evolution of this early deepwater slope system was controlled largely by short-distance aeolian transport of very fine sediment from the coast.

Heavy mineral incorporation into Brushy Canyon Formation slope deposits as reflected in laminae-scale bulk Zr and Ti abundances may preserve a long-term record of local wind intensity during the Middle Permian. If so, then relative Zr and Ti abundances should be correlatable between different vertical sections. A PhD student, Ms. Kannipa Motanated, is currently measuring heavy mineral abundances in siltstones collected from several sections below and above the Salt Flat Bench sandstones to test this prediction.

Turbidite A Divisions

Zircon and rutile grains in turbiditic structureless sandstones (Bouma A divisions) of the Brushy Canyon Formation have calculated terminal fall velocities significantly less than associated feldspar grains, despite the fact that all three grain populations are very well sorted. Indeed, standard deviations of grain sizes for these populations are statistically indistinguishable, suggesting that they all had identical fall velocity distributions. The most likely explanation for the discrepancy in calculated fall velocities is that all grains settled through a dense suspension of sediment, and that the lighter and larger feldspar grains were more hindered by grain interactions than the denser and smaller zircon and rutile grains. Preliminary calculations based on effective medium theory suggest that Brushy Canyon Formation turbidity currents were highly concentrated, with suspensions more than 15% sediment by volume. Ms. Motanated is leading a team of undergraduate researchers in refining these calculations using settling experiments.

Early Carbonate Cementation of Organic-Rich Turbidite Tops

Carbonate cements are common in organic-rich sedimentary rocks and significantly modify rock porosity, permeability, and fracture mechanics in both conventional and unconventional reservoirs. A variety of anaerobic microbial metabolisms are known or suspected to promote carbonate mineral precipitation. However, controls on microbially-promoted cementation are poorly understood, limiting our ability to construct process-based models of diagenetic facies or to understand interactions between diagenesis, transport, and deposition. Channel-filling turbidites in the study area show evidence for incomplete erosion of laminated siltstones (Bouma D divisions) through iron- and manganese-rich partially burrowed tops down to dolomite-cemented, pyritic bases. In the relatively rare instances where D divisions were completely eroded, underlying A division sandstones were also eroded down to the next D divisions. Cemented intervals in D divisions are up to 60% cement with bulk carbonate isotopic compositions implying formation at low temperature with very little incorporation of respired organic carbon. These observations suggest that carbonate cements formed in the anaerobic zone, and most likely in the iron reducing zone, of reducing turbidite tops during intervals when the overlying water column was weakly oxic. These cements modified sediment transport and deposition patterns in the mid- to upper-slope. If widely developed, similar cements in other settings would play a role in determining rock properties of reservoirs deposited in deep-water settings. The cementation model is now being tested experimentally by a Ph.D. student, Mr. Zhirui Zeng, in the PI’s lab.

Impact on PI’s Career

This grant has allowed the PI to branch into several new areas of sedimentology with application to petroleum geology, and has already spurred contacts with industry that have led to access to new core for other research projects and potential future sources of funding. The model of carbonate cementation developed as part of this project is now being pursued by a MS student in an active gas shale play. It has indirectly led to collaborations with researchers at other universities in areas well outside of the PI’s previous area of expertise.

Impact on Students’ Careers

Mr. Gunderson reports that this grant has given him credible experience in a petroleum-productive basin that has allowed him to successfully pursue an internship and a long-term job offer from a petroleum company. Ms. Motanated has received direct financial support from this grant. Her career goal is to become a professor in petroleum and sedimentary geology. Mr. Zeng is currently supported by this grant and using the experience to transition from a microbiology MS to a petroleum-relevant PhD.

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