| dc.description.abstract | The Late Cretaceous Austin Group contains a petroleum play fairway that spans across east and south Texas. This unit is composed of shallow to deep-water carbonate and marl, which were deposited along the southeast margin of the Cretaceous Interior Seaway, during the latest Turonian through earliest Campanian. Giddings Field in east-central Texas is one of the most prolific oil and gas fields producing from the Austin Chalk. As such, Austin research to date in east-central Texas has focused on the immediate Giddings Field area. However, the area west of Giddings Field, in Bastrop, Fayette, and Lee Counties, where the Austin Group approaches the San Marcos Arch, has received little subsurface scrutiny in comparison to the Giddings area to the east, or the shallower subsurface and outcrop regions toward the north. Through detailed examination and correlation of wireline well logs and a partial core, a new understanding of the chronostratigraphic framework defining depositional and post-depositional influences in the distribution and thickness variations within the Austin Group was determined. This new understanding sheds light on potential petroleum reservoir and play distributions across the study area.
The Austin Group in the study area is unconformably bound by the Eagle Ford Group below and Taylor Group above. Within the Austin Group a major maximum flooding (downlap) surface, and underlying Sequence Set Boundary (SSB), were used to sub-divide the Austin Group into two major sequence sets. These sequence sets respectively define the Lower Austin Chalk (LAC), which is interpreted as a transgressive sequence set, and Upper Austin Chalk (UAC), which is interpreted as highstand sequence set.
The LAC is bound at the base by the Austin Group and Eagle Ford Group contact and above by the SSB. It is composed of four high-frequency sequences (S1, S2, S3, and S4), readily identified by their associated maximum flooding surfaces. These high-frequency maximum flooding surfaces, along with the upper and lower LAC boundaries, were used as correlation markers that constrained five correlation units within the LAC. These correlation units indicate the westward thinning of the LAC is a result of onlap at the base and stratal convergence to the west. The LAC correlation units also highlight a central paleo-topographic high in the area identified as an ancillary, or relict feature of the San Marcos arch. The depositional influence of this ancillary high is most evident in the lower units of the LAC, indicates minimal influence in the overlying LAC units, and has no influence directly above in the UAC units. The San Marcos Arch proper expresses a dramatic depositional influence further west in the UAC, where there is little to no influence directly below in the LAC.
The UAC is bound by the SSB at its base, and the unconformity at the base of the Taylor Group at its top. The UAC is composed of eight high-frequency sequences (S5 - S12), readily defined by the associated eight high-frequency maximum flooding surfaces. The uppermost seven maximum flooding surfaces, combined with the UAC upper and lower boundaries, were used as correlation markers that constrained eight correlation units within the UAC. These units thin considerably as they approach the San Marcos Arch to the west and lap-out on to the downlap surface directly above the SSB in the middle of the Austin Group. The east-southeast corner of the study area records substantial eastward thinning of the UAC caused by the erosion of the five uppermost units through incision from the overlying Waco Channel, which defines the base of the Taylor Group in the eastern part of the study area.
Onlap of the lower LAC units against the ancillary San Marcos Arch at the base of the Austin Group, downlap of UAC units just above the SSB in the middle of the Austin Group, as well as truncation beneath the Waco Channel at the top of the Austin Group, control the thickness and distribution of the various chronostratigraphic units within the Austin. These factors impact the plays, play fairways, and traps within the Austin Group where units terminate against the Waco Channel, San Marcos Arch, the paleo topographic high in the LAC, or where structural highs occur, and need to be considered for further exploration and exploitation activities within the study area and beyond. | en |
