Identification of sandstone-rich zones in upper bathyal, deep water environment on south Texas Gulf Coast

Abstract

We document the results of a 3D seismic investigation undertaken in search of prospective hydrocarbon, sandstone-rich zones within the Eocene Yegua Formation, which is located in a shale-dominated environment in Jackson County, South Texas. Because of the preponderance of shale in this environment, diligent effort and methods were required to identify sandstone-rich zones. First, we used seismic sweetness and instantaneous frequency attributes to identify sandstone-rich zones. This procedure was then followed by a frequency-fusion method to validate results and estimate the thickness of predicted sandstone-rich intervals. Results show that these zones are characterized by low frequencies ranging from 0 to 15 Hz. We interpreted the depositional pattern to be slope channels and slope fans deposited in an upper bathyal environment, trapped by down-to-the-basin synthetic growth faults and shale ridges. The growth faults found within this Yegua deep water setting influence the sandstone-rich zones in two ways: (1) they act as a barrier against which sandstones are trapped and (2) they downfault sediments to deeper water, making recognizing the continuity of the channels difficult. On the basis of frequency fusion, we infer that the thickness of the sandstone-rich zones ranges from 30 to 45 m (100–150 ft). Although sweetness, frequency fusion, and instantaneous frequency attributes can help in identifying sandstone-rich zones in shale-dominated environments, a combination of these attributes and structure is required to determine whether the sandstone-rich zone could be a potential prospect.