Separating structural from depositional complexity in the Paleocene Lower Wilcox "Lobo" Sands, Zapata County, Texas
The Lobo Trend of Webb and Zapata Counties, Texas, lies in the northern part of the Gulf of Mexico Basin and is part of the Lower Wilcox (Late Paleocene) stratigraphic unit that is observable across South Texas. The informally named slumped and faulted deposits of the lowermost Wilcox Group, the Lobo Trend, is comprised of wavedominated deltaic and shoreface deposits which consist of various lobes of well developed fine-grained, tight sandstone. Subsequent structural complexity makes the unraveling of the original sedimentary complexity a challenge. The Lower Tertiary deposits of the Lobo series average 800 feet thick (Claughton, 1977) and represent progradational shallow-marine shoreface sands. These deltaic and shoreface sands were part of larger deltaic systems with sediment derived from the north and west (Fisher and McGowen, 1967; Galloway et al., 2000). Through both fluvial and longshore transport sediment accumulated as shoreline sands in the mid- to outer-shelf in the Gulf of Mexico Basin. These sands subsequently underwent severe disruption and faulting in response to large-scale postdepositional slumping. In many locations the Lobo sandstones provide a quality reservoir rock for oil and gas accumulation. More than 4.5 Tcf of natural gas have been produced from the sands of the Lobo Trend (Anno et al., 2002), and development operations continue today, making the Lobo of great interest to many individuals and companies. Stratigraphic and sedimentologic studies on the area, however, are severely hampered by structural intricacies, and as a result, studies of the depositional history and stratigraphy of the area have been limited. This study focuses on the main Lobo sand lobe packages present in northeastern Zapata County, Texas. The dataset consists of 3-D seismic coverage, well logs, thin section samples, and core data. Stratigraphic and structural cross-sections have been generated from the integrated dataset to help define the depositional history and stratigraphic framework of the study area and sequence stratigraphic concepts from both the Galloway model and the Vail (Exxon) model have been applied. To evaluate possible provenance of these sands, a petrographic study was performed. Additionally, a workflow model has been designed for the interpretation of this data, with the aim of separating the structural from the depositional components of complexity, thus providing further regional understanding and prediction of the Lobo series in northern Zapata County.