2025 GCAGS Journal, Vol. 14
TITLE
Stratigraphic Evolution of Fluvial-Dominated Deltaic and Wave-Modified Shoreline Systems in the Eocene Yegua Formation in Northern Jim Wells County, South Texas
AUTHOR(S)
William A. Ambrose and Harold H. Rogers, III
ABSTRACT
The late middle Eocene Yegua Formation in northern Jim Wells County in South Texas is a stratigraphic succession of fifteen fourth-order depositional units composed of fluvial-dominated delta, wave-dominated delta, and shoreface deposits. These depositional units collectively compose an offlapping, progradational clastic wedge punctuated by two major retrogradational cycles. During periods of coastal offlap, a succession of fluvial-dominated deltas prograded eastward and southeastward across northern Jim Wells County. In contrast, retrogradational cycles are marked by strike-elongate, northeast-trending net-sandstone trends that record wave-reworked deltaic headlands and continuous (>25 mi [>40 km]) sandy belts composed of wave-dominated shoreline deposits.
Distributary-channel deposits in the Yegua Formation are defined by narrow (<2 mi [<3.2 km]), dip-elongate and southeast-bifurcating net-sandstone trends with ≥30 ft (≥9 m) of net sandstone. The transition from prodelta to proximal-delta-front and channel-mouth-bar facies, interpreted from whole-core data, consists of 40 to 50 ft (12 to 15 m), upward-coarsening sections of burrowed, silty mudstones interbedded with thin (<1 ft [<0.3 m]), very fine-grained sandstone beds, in turn overlain by 10 to 12 ft (3 to 6 m) sections of very fine- and fine-grained sandstone with plane beds and ripple cross-stratification.
Oil and gas production in the Yegua Formation in northern Jim Wells County is primarily related to fault-bounded, sandy distributary-channel, delta-front, and upper-shoreface deposits. Major Yegua oil and gas fields include Alice, Amargosa, Ben Bolt, Braman, Muerto Creek, Orange Grove, and San Diego East fields. Primary producing facies in Yegua fluvial-dominated deltaic systems include distributary-channel margin, distributary-channel, and channel-mouth-bar/proximal-delta front, with production in individual gas wells ranging from <100 MMcf (million cubic feet) to >1 Bcf (billion cubic feet). Oil production in the Yegua Formation in northern Jim Wells County is comparatively minor compared to gas production, with the greatest cumulative production in many individual wells <20,000 bbl (barrels). However, oil production in some wells exceeds 100,000 bbl. Oil and gas production in Yegua wave-dominated shoreline systems is less than in fluvial-dominated deltaic systems, with cumulative oil production in most individual wells <5000 bbl. There are numerous areas in northern Jim Wells County where potential reservoir facies in the Yegua Formation could be tested for additional oil and gas production. These areas occur where regional faults transect narrow and sandy distributary-channel deposits and where sandy, wave-reworked deltaic headland deposits pinch out into delta-fringe mudstones.
PAGE(S)
36-58
DOI
https://doi.org/10.62371/YAYW3313
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