Abstract

Subtle traps are usually thought of as those which are not easy to identify as anomalies on conventional seismic sections and subsurface geological maps. Many such features—the East Texas Field, for example—are classified as stratigraphic traps, but they are greatly dependent on paleo-topographic relief, presently expressed as isopachous thins, for their origin. Good examples of paleostructural influence on trap and reservoir origin are present in the Cotton Valley Formation of East Texas (Jurassic), the Chappel Formation (Mississippian), and the Palo Pinto Formation (Pennsylvanian) of North Texas. Studies of Cotton Valley Lime reservoirs at Teague Townsite Field in Freestone County, Indian Rock—Gilmer Field in Upshur County, and Overton Field in Smith County reveal a peculiar type of early diagenetic microporosity that formed only on the crests of salt dome structures or basement features which were topographically high at the time of Cotton Valley Lime deposition. Jurassic-aged topographic highs over salt domes commonly do not align with present structural closure; consequently, the early diagenetic porosity may lie off the present structural crests. Other, post-Jurassic salt dome highs appear as present structural anomalies but are non-porous. The key to finding these subtle traps are isopach maps of the overlying Bossier Shale. Thin Bossier Shale indicates an underlying high of microporous Cotton Valley Lime. Similar examples exist at Conley Field in Hardeman County where both the Chappel and the Palo Pinto formations produce from reservoir/trap combinations that formed on the crest of the pre-Mississippian high. The Chappel Limestone occurs as a series of bioclastic, carbonate sand waves stacked up like wind-rows on top of the paleo-high. Reservoir porosity is mainly depositional in nature. Bryozoan fragments makeup much of the bioclastic sand and are themselves porous, creating the main reservoir porosity. The trap exists as a facies change: no bryozoan sands—no reservoir. Finally, the Palo Pinto Formation also produces at Conley Field from a reservoir that consists of leached, moldic, and vuggy porosity in a platy algal reef facies that developed only on the crest of the underlying high. Trapping facies are present off the paleohigh and in overlying strata. The paleostructure at Conley Field coincides with present structure, and the field could have been found without using isopach maps. However, there are many structural anomalies in the Hardeman, Fort Worth, and Palo Duro Basins that have been drilled as dry holes. The key to these subtle traps is an isopach map to show that there was a preexisting high present to seed reef growth and bioclastic sand accumulation. Present structural anomalies which lack the ancient structural component also lack reservoirs.