Trap and Fault Analysis: Revisiting Texas Gulf Coast
The Texas Gulf Coast is undergoing another hydrocarbon exploration resurgence, with many areas being reworked with new seismic processing methods and attribute analysis. Explorers are again searching for conventional structural and stratigraphic traps. Using “something old to find something new”, fault seal and trap analysis is applied to better understand trapping and charge. Arguably the most important contribution to fault seal analysis has been provided by U.S. Allan’s 1989 paper, Model for hydrocarbon migration and entrapment within faulted structures (AAPG bulletin). Much of the background on this paper and others were based on case studies from the Gulf Coast.
When risking fault bounded structural traps, across-fault juxtaposition and/or fault membrane seal are key issues. A large number of studies have considered membrane sealing as the primary trapping element. Many engage in back calculating the pressure capacity by using a Shale Gouge Ratio (SGR) algorithm, and then forecast hydrocarbon column height. Importantly, this back-fitting of SGR and seal capacity is almost always conducted using single “best” technical models without modelling uncertainty. Fault and stratigraphic uncertainties are significant and need to be explicitly included in the modelling of fault seal risk and inferred column heights.
In general, SGR methods in systems that have moderate Vshale values artificially increase predicted column heights and enhance the pre-drill estimates of success relative to Allan’s juxtaposition analysis of cross-fault leakage. Operators using SGR often discount these large columns through other geologic risk factors. When shorter columns are found during drilling, they are often “explained” by issues of charge or trap breach.
Although much of the current exploration effort in the Gulf Coast is focused on stratigraphic trapping, it is still important to understanding the fill and column control for older discoveries. A series of case studies will be presented in which observed hydrocarbon water contacts are compared with probabilistic fault seal models. World-wide studies show that probabilistic juxtaposition models more accurately predict hydrocarbon water contacts than calibrated SGR single “best” technical models. Importantly these studies affirm Allan’s work, illustrate that accumulations are generally filled to their capacity, either due to structural spill or fault leak points.
AAPG Datapages/Search and Discovery Article #90350 © 2019 AAPG Annual Convention and Exhibition, San Antonio, Texas, May 19-22, 2019