Texas produces about 43 percent of crude oil and 29 percent of natural gas in the US; it ranks number one. The geology of Texas also exhibits important outcrops that shed light on subsurface formations. This edition of Core Elements highlights the outcrop geology of Texas through some recent publications.
Creators: Author Nathalie Brandes and photographer Paul Brandes are wife and husband geologists.
Geologic divisions of Texas: The book divides Texas into six geologic zones: (1) High Plains, (2) North-central Plains, (3) Coastal Plains, (4) Central Texas (Llano Uplift), (5) Edwards Plateau, and (6) Basin-and-Range.
Divisions 4 and 5 lie totally within Texas, while the other geological zones extend to the neighboring states of Oklahoma, New Mexico, or Louisiana.
Contents: After an introductory chapter on the geological history of Texas, 80 geological sites are described, each site on two pages, including location maps and field photographs.
Geological sites include the oldest to present-day, such as:
Coal Creek Serpentinite (Grenville age)
Packsaddle Schist in Llano (Grenville age)
Guadalupe Mountains National Park exposing Permian reefs
Padre Island: Longest barrier island in the world
1901 Spindletop oil gusher
Go deeper: Texas ROCKS! is a great guidebook to interesting geological sites in Texas.
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Fracturing in Chalk-dominated Carbonate Reservoirs
Anacacho Limestone/David Ferrill
An article in the Journal of Structural Geology reports on an outcrop study of fractures in a chalk-dominated carbonate formation in central Texas.
About chalk:
Chalks are white, often porous, carbonate rocks with generally low matrix permeability.
Their porosity and permeability depend on burial compaction and cementation.
Fractures play an important role in the permeability of chalk.
Anacacho Formation: A Cretaceous age carbonate formation exposed along the Balcones fault system – a series of normal faults between the Edwards Plateau and Coastal Plains.
Study Design:
The researchers conducted field work and collected fracture data along two transects of Anacacho Limestone near and far from the Balcones fault.
Drone photogrammetry also helped with gathering structural information from the study area.
Rebound Schmidt (Schmidt Hammer) tests were performed on the rock samples as a proxy for rock mechanical properties, particularly relative stiffness or competency of the rock.
Rock samples were also analyzed for X-ray diffraction (XRD) mineralogy.
Samples of calcite veins were analyzed by U-Pb dating and fluid inclusion method.
Study Results:
U-Pb dating showed that the Balcones faulting was active mainly between 58 to 39 Ma and to a lesser degree at 17 Ma. These dates add new information to a previous U-Pb geochronology by Hippolyte et al.
Sedimentary beds with high rebound numbers (more comprehensive strength) that contain less than 7 percent clay and more than 90 percent carbonate display well-developed open fractures. Low mechanical rebound samples lack these features.
Fractures have predominantly a NE–SW strike consistent with the regional normal-fault strike.
Near-fault fracture intensity reaches five times the background levels; fracture intensity diminishes to background levels about 30 m from the fault.
Away from mapped faults (with throws of ~10 m or greater), structural deformation is represented by opening-mode fractures in two orthogonal sets with intensities of no more than one fracture per meter.
Fluid inclusions of calcite-filled fractures revealed oil inclusions with 28-36 API gravity and homogeneous temperatures from two-phase inclusions corresponding to burial depths of 0.9 to 1.5 kilometers for oil inclusion and 2.4 to 2.9 kilometers for brine inclusion.
Why it matters:
Chalk-dominated formations are important for oil and gas reservoirs, aquifers, and potential subsurface carbon-storage sites.
Fracture characteristics of chalk are key petrophysical information for fluid flow.
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Fracture Study of Wolfcampian Alta Formation in the Marfa Basin
Alta formation/David Ferrill
The Delaware and Midland basins of the Permian superbasin are leaders in the US shale oil and gas revolution. The Marfa basin, the third basin of the Permian superbasin located to the southwest of the Delawar basin, is perhaps a sleeping giant.
A study published in Marine & Petroleum Geology describes rock fractures in the exposed Alta formation of the Marfa basin.
Study Area:
The Marfa basin has undergone a more complicated tectonic history than the Delaware and Midland basins because of its involvement in the Laramide orogeny (Cretaceous-Paleogene age) and Basin-and-Range extension (Oligocene-Miocene age).
Chinati Mountains were produced by magmatic activity during the Oligocene.
The Alta formation comprises deepwater siliciclastic sediments of Wolfcampian (Early Permian) age.
About 1,650 meters of the formation is exposed in the southern part of the Chinati Mountains.
Study Design: The study included field mapping, drone photogrammetry, structural data collection, XRD mineralogy, and organic carbon measurement.
Study Results:
Fracture sets: Structural deformation at the reservoir scale consists of four major systematic sets of bed-perpendicular opening-mode fractures.
Fractures within strata: Opening-mode fractures are generally restricted to their host sedimentary beds, rather than cutting across multiple beds.
Fracture length and height: As measured in a sandstone bed, the ratios of fracture length (parallel to bedding) to fracture height (perpendicular to bedding) for opening-mode fractures range from 0.13 to 38.56 with an average ratio of 4.84.
Mineralogy impact: Fracture spacing of NE-striking opening-mode fracture sets shows a positive correlation with clay content and an inverse relationship between fracture spacing and quartz-feldspar content.
Bed thickness: Fracture spacing vs. bed thickness shows a strong positive correlation for sandstone beds and an inverse relationship for shale beds.
Rare structures: Less frequent structures include normal and thrust faults and bed-parallel “beef” veins of fibrous calcite.
Why it matters:
The Alta formation is similar in stratigraphic age, rock type, and depositional environment to the prolific Wolfcampian formation in the Delaware and Midland basins.
The outcropped Alta formation offers a field analog for corresponding subsurface reservoir rocks.
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