Gulf of Mexico Inboard Lower Tertiary and Cretaceous: Plays and Potential
Recent lower Tertiary and Cretaceous onshore Gulf of Mexico discoveries have re-focused the venture capital cross-hairs on conventional and semi-conventional opportunities. This trend had started prior to the most recent price crash, but conventional oil and gas prospects steadily became more appealing versus unconventional drilling starting in early 2015. Enhanced subsalt seismic imaging combined with improved completion techniques in extreme temperature environments, has potentially added trillions of cubic feet of new gas reserves to a very mature basin.
For example, in early 2015, Freeport-McMoran Oil and Gas (FM O&G) discovered 150 ft of net Cretaceous pay at depths of 24,000–29,000 ft in St. Martin Parish, Louisiana. The first production from a subsalt well in south Louisiana, the Highlander discovery tested multiple Cretaceous sands at 75 million cubic ft of gas per day, and seismic suggests the reservoir could encompass ~28,000 ac, or potentially 2–3 trillion cubic ft of resource. This deep subsalt play is characterized by high pressures and temperatures, and resembles a ‘turtle’ structure partially trapped against an old salt uplift.
The Highlander success is directly linked to the enigmatic 2009 Davey Jones well, drilled just offshore in South Marsh Island Block 230 by FM O&G. Louisiana is synonymous with serendipity, and although Davey Jones was exploring for deep Wilcox sands, they found hydrocarbon bearing Lower Tuscaloosa sands below 29,000 ft that exhibited anomalous 15–20% porosities. This deeper revelation helped lead to the Highlander triumph, potentially unlocking a much larger treasure chest than the porosity challenged Tertiary targets.
Depositionally, these enormous volumes of Cretaceous clastics were sourced from the erosion of the Ouachita and Appalachian uplifts and then funneled into the Gulf of Mexico by way of the Mississippian Embayment. Not only do the newly drilled Highlander and Davey Jones wells prove this point, but in 2001, Shell discovered over 980 ft of (tight) Tuscaloosa-aged rocks 250 mi from the established Cretaceous shelf edge in Alaminos Canyon Block 557.
These enormous volumes of host rock take second chair to the dynamic digenetic history which has produced reservoir grade porosities at depths greater than 30,000 ft. It's common knowledge that given the right temperatures and pressures, the dissolution of volcanic rock fragments can create secondary porosity. This process also supplies the right ingredients to form chlorite, which in turn prevents quartz overgrowths and maintains original primary porosity. The resulting high porosities could potentially extend significantly further basinward than originally theorized, setting up a new resource trend which could prove up tens of trillions of cubic feet of gas.
AAPG Datapages/Search and Discovery Article #90219 © 2015 GCAGS, Houston, Texas, September 20-22, 2015