ABSTRACT: Computer-Aided Mapping of the Pre-Hackberry Unconformity in Northeastern Calcasieu Parish, Louisiana

Thomas C. Chandler, Gary L. Kinsland

The pre-Hackberry unconformity is an important stratigraphic feature that, while recognized, has not received the attention due such a significant horizon. It is at least regional in extent, as it underlies the Hackberry Embayment of southeastern Texas and southwestern Louisiana, and there is current speculation that it could be part of a larger interregional unconformity associated with a major eustatic sea-level fall in the middle Oligocene. It is a proven hydrocarbon trapping horizon in Beckwith Creek field of Calcasieu Parish, Louisiana, yet little effort has been made to exploit this trapping mechanism elsewhere. Unfortunately, it suffers the same stigma of unpredictability as the sand facies of the overlying basal Hackberry turbidites, which is very erratic. However there is a group of more predictable reservoir sands that is also associated with the pre-Hackberry unconformity. They are pre-Hackberry, but post Nonion Struma beds that have been truncated by the unconformity (Beckwith Creek field produces from these beds). Recognition of the fact that the basal Hackberry turbidites lie immediately above, and the post Nonion Struma sands immediately below, the unconformity along with accurate differentiation between the two sections are keys to identification and mapping of the unconformity. Detailed mapping of the unconformity can pinpoint areas of interest such as highs that have potential for production from the underlying post Nonion Struma sands, and lows that have potential for production from the overlying ponded and/or channel sands of the Hac berry turbidite section.

Computer graphic programs have been utilized for rapid creation of preliminary graphic displays of the pre-Hackberry unconformity. With inputs of only map loci and depths to the unconformity, the computer created preliminary contour maps and three-dimensional perspective diagrams that provided visual evidence of general surface trends, apparent fault escarpments, possible channel cuts, and other structural features. These computer-generated surfaces have been used as guides in more detailed and comprehensive follow-on studies. The results of our studies illustrate that viable computer graphic products, made possible by accurate identification of the pre-Hackberry unconformity, may be the impetus needed to generate interest in this important but mostly overlooked horizon.

AAPG Search and Discovery Article #90999©1990 GCAGS and Gulf Coast Section SEPM Meeting, Lafayette, Louisiana, October 17-19, 1990