A Stratigraphic and Geochemical Analysis of the Ojinaga Formation, West Texas
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Date
2016-07-28
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Abstract
Stratigraphic, biostratigraphic and geochemical analysis yields a greater understanding of the depositional environment of the Upper Cretaceous Ojinaga Formation in West Texas. A detailed measured section of the Ojinaga Formation in Hudspeth County can be correlated to the coeval Boquillas Formation in the Big Bend Region (Brewster County, West Texas) and prolific unconventional Eagle Ford Group in Terrell County (West Texas), providing a better understanding of the Cenomanian-Turonian depositional system in west Texas.
A high-resolution study of two outcrops in Mule Canyon in Hudspeth County, West Texas, documents this unit’s lithology, composition, sedimentary structures, and gamma ray response on a local scale. Three distinct lithostratigraphic facies, termed facies 1-3 from the base up were identified in the Ojinaga Formation based on significant lithological and geochemical changes. The most notable change in facies is the transition from facies 1 to facies 2, indicating a depositional shift from storm [influenced sediments on a carbonate ramp to deep ramp sediments as the result of an increase in accommodation. Facies 2 represents the greatest thickness observed in the section, suggesting an extended period of marine transgression and high accommodation. Facies 2 gradationally transitions into facies 3, noted by increasing carbonate-dominated deposits and decreasing pelagic mudstone. The increase in frequency of carbonate beds interbedded with mudstone may suggest a marine regression and decrease in accommodation.
Geochemical analysis supports the interpretation of three facies within the Ojinaga Formation in Mule Canyon. It is the geochemical observations derived from spectral gamma ray response used to correlate the Ojinaga with the Boquillas and Eagle Ford formations. From this work it is evident that the Ojinaga Formation at Mule Canyon correlates almost entirely to the Lower Eagle Ford Formation in Terrell County. Only a few meters of the uppermost portion of the section at Mule Canyon can be correlated to the Upper Eagle Ford Formations. Biostratigraphic data, indicating that the Mule Canyon Ojinaga Formation strata are Cenomanian in age, supports the gamma ray correlations.
The Ojinaga Formation records a shift in depositional environment from lower ramp carbonate tempestite deposits to more basinal sediments. A change from hummocky to swaley cross-stratification with bedding scour bases, to organic rich carbonate-dominated mudstones are the lithological reflections of this shift. Geochemical analysis indicates the Ojinaga Formation was deposited during euxinic (likely episodic) periods due to basin restriction with increasing marine circulation and oxygenation occurring over time toward the top of the section. Unique environmental conditions in the Western Interior Seaway during portions of the Middle to Late Cenomanian allowed for more total organic carbon preservation in the lowermost portions of the Ojinaga Formation.
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Carbonates, Unconventional, Petroleum, Sequence Stratigraphy, Cenomanian