A big missing piece to the puzzle that is the still-fledgling Cretaceous-age Tuscaloosa Marine Shale (TMS) play in central Louisiana and southwestern Mississippi is the overall lack of published studies on it.
The familiar 1997 study, which originated at what is now the Basin Research Energy Section of the Louisiana Geological Survey, continues to be the defining work.
That’s why the work of AAPG member John Allen, geologist at Tesla Offshore, New Orleans, might be of keen interest to geologists. He elected to write about the TMS for his recently completed thesis at the University of Southern Mississippi.
Allen received his master’s degree there in geology in 2013, so the publication is hot off the press.
“A friend of mine was working in the shale and suggested the topic,” Allen said, “and my professor thought it was a good idea, and that’s how I got into it.”
The title says it all: Determining Hydrocarbon Distribution Using Resistivity, Tuscaloosa Marine Shale, Southwestern Mississippi.
He emphasized that the purpose of the study was to investigate the TMS Trend in southwestern Mississippi using electric logs and IHS’s proprietary Petra mapping software to document the structure, stratigraphy and hydrocarbon distribution.
The maps and cross sections generated include:
♦ Isopach map of the base of the resistive section to the first Lower Tuscaloosa sand.
♦ Net pay isopach map of the resistive section.
Allen noted that the paleogeography of the study region undoubtedly provided the perfect scenario for geologic development of the TMS resource.
“It provided the necessary depths required for the rocks to mature,” he said. “And it provided the accommodation space needed to guide the Tuscaloosa deltas into the area, which may have enhanced the preservation potential of organic matter through periodic rapid burial, and deposited the cross bedded laminations found within the section.”
It’s All Good News
Allen elaborated, pointing out that the laminations produced structural planes of weakness, which intensified fracturing, porosity and permeability, tending to make the rock more brittle and susceptible to additional fracturing.
The array of conclusions reached from the study likely will pique the interest of anyone playing, or thinking about playing, this still-new frontier.
He noted, for example:
♦ The resistive, hydrocarbon-bearing section of the TMS is located at the base of the shale, tending to thicken with depth throughout the study area.
♦ The areas of thickest high resistivity are located within the southern part of Wilkinson County, where thickness tallies 210 feet. This is triggered by higher temperatures and pressures occurring with the increased depths, which enables the organic-rich rock to mature, spurring further hydrocarbon generation.
♦ Gas volume and pressure increase with depth throughout the study area, as indicated by mudlogs.
“These greater pressures along with the gas will stimulate the oil trapped in the tight shale to travel to the surface during production, greatly increasing the chances of attaining commercial quantities of hydrocarbons,” Allen said.
Enter a caveat:
“The resistive section lies directly above the Tuscaloosa sands, and this could lead to potential hydraulic fracturing risks,” he cautioned.
At the end of the day, it’s all good news.
“This study identifies areas favorable for hydrocarbon production,” Allen said, “while indicating areas potentially hazardous to hydraulic fracturing.”