Monday, July 8, 2019

Regional Geology - Understanding the Basin

I believe that one of the key factors for most of the "capital destruction" in Shale 1.0, "The Great Land Rush", was due to two factors.  The first is that the land teams got too far ahead of their technical team's evaluation.  Very large acreage blocks were at risk of being lost so the company chose to invest in the acreage before their technical team had completed a thorough analysis.  I recall a story in Colorado where the land team of a very large independent leased an area where the Niobrara was outcropped at the surface.  Field trip opportunity for the geos!

The second reason is that the technical teams only focused on the "zone of interest" and spent little time gaining an understanding of the basin as a whole.  The regional structural, stratigraphic, and geochemical framework is so important.  The formations below and above the zone of interest provide vital details. 

This phenomena played out in the TMS during its first run from 2010-14.  Mudlogs and resistivity curves were the focus for many.  Now with interest increasing in the Austin Chalk, I'm not sure if many have invested the time to understand the basinal configuration and history.  The old science of "trendology" appears to be the "tool of choice" to date in the LA-West region. 

Austin Chalk 3.0 in Texas is being driven by the mapping of matrix porosity.  The current "hot spot" for drilling in Texas is located where horrible results were found during Austin Chalk 2.0 in the 90's.  Understanding the spatial distribution and thickness of porosity, saturation, geomechanical, and geochemical parameters is now the key to this current puzzle.

The maps below provide some regional views incorporating significant geological structures, salt features, bouguer gravity, and a simplified structural map on the Base of the Austin Chalk.  In the early 90's while at Amoco, I spent five years evaluating this basin incorporating paleo, 2D/3D seismic, logs, production, conventional core, and drill cuttings.  The most significant aspect across this basin is understanding the impact of the different Lower Cretaceous shelf edges and the prominent role that salt played with regards to deposition and structure.  Updip and downdip of the Fredericksburg Shelf Edge are two "different worlds" geologically.  The bouguer gravity map below illustrates the major difference on strike between LA-West to LA-East.  The ancestral Mississippi River created a very different depositional history in LA-East compared to LA-West.

Salt ridges and piercement domes greatly impacted the depositional patterns of the Tuscaloosa sands.  The most prominent Tuscaloosa field, Port Hudson, sits on top of a piercement salt dome.  Salt ridges guided the Tuscaloosa fluvial systems into the basin.  For the Austin Chalk, it will be interesting to see how salt-induced fracturing impacts the frac jobs and producibility of the formation.  Some fracturing combined with good matrix porosity should be a good combination.  Too much natural fracturing combined with a large proppant frac might connect with multiple water sources negatively impacting production.  These structural aspects will be important to evaluate as the results come in.












BOUGUER GRAVITY - Source: USGS https://pubs.usgs.gov/ds/352/arkla_bou.html

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