Thursday, March 28, 2019

Regions - Geological Comparison and Contrast

In my November 5, 2018 post, I presented detailed maps illustrating geographic regions across the Austin Chalk Trend.  This allows for comparison and contrast across this vast trend.

The table below provides a comparison and contrast of the geographic regions of the Austin Chalk in Louisiana and Mississippi.  Due to the fact that I've not evaluated the Texas Austin Chalk trend to the same extensive detail that I have in Louisiana, I'm only presenting the table for LA and MS.

The table subdivides various attributes into the categories of geology, hydrocarbons, and log analysis.  At the top, the range in depth for the base of the Austin Chalk is presented.  The updip limit of my region line is not based on any geochemical maturation data, but is purely based on a consistent distance from the shelf edge.  The top line does align with the shallowest production in Giddings Field (Texas).  The updip limit will be defined in the future as wells delineate this important termination point.  At some point updip, the Austin Chalk of Louisiana/Mississippi will be immature.  It's very likely that the updip limit varies across the play due to variations in burial depth and local tectonics.  Giddings Field in Texas (TX-Central) produces as shallow as -5200' (SSTVD).  In Texas the Eagle Ford provides most of the hydrocarbon source.  The downdip limit across the entire play has yet to be defined.  Pressure, temperature, costs, and economics will be the limiting criteria at some point.

Today I will focus on the geological aspects of the play. One of the major contrasts between the updip and downdip fairways in Louisiana is the geological structure.  I use the Lower Cretaceous (Fredericksburg) Shelf Edge as the defining boundary for updip and downdip.  The updip region exists upon a very stable Lower Cretaceous platform which results in a simpler and "quieter" geological structure.  The region exhibits gentle monoclinal dip with limited faulting.  Drilling and staying "in zone" will be much easier in this region.  Dip occurs in the southwestern direction.

Example of the structure in the Updip Region (Source: Amelia Resources LLC)

South of the Fredericksburg Shelf Edge in the downdip region, the basinal setting presents an environment for major subsidence, accommodation space, depositional faulting, and salt tectonics.  South of the shelf edge is a very typical Gulf Coast basin with 3-way and 4-way structural closures.  Structural dip in a given fault block can occur in any direction as a result of the complex depositional faulting.  These structures trapped and produced 1.4 trillion cubic feet equivalent of natural gas from the expanded Tuscaloosa Sands.

Example of the structure in the Downdip Region (Source: Amelia Resources LLC)

At the Austin Chalk level, most faults are strike trending and parallel the shelf edge with splinter faults oriented in oblique directions.  Continued subsidence after the deposition of the Tuscaloosa clastics, presents large faults displacing the Austin Chalk with throws ranging from 50-500'.  The orientation, spacing, and throw of these faults can present significant challenges to drilling a horizontal wellbore that has a 5,000-10,000' lateral.  Crossing over a 250' fault wreaks havoc on geosteering and zone placement. 

I believe that the most important risk in the downdip region is the risk of fracking into the faults and natural fracture clusters connecting to large water sources.  In the downdip region of LA-WEST, Masters Creek, Burr Ferry South, and Cheneyville produced on average 6.29 barrels of water per barrel of oil.  Many wells in Masters Creek had ratios as high as 10-35.  In contrast, Burr Ferry North in the updip region produced 0.5 barrels of water per barrel of oil. The same low water ratio is seen on trend in the northern updip portion of Brookeland Field in TX-EAST.  A large proppant frac in the Masters Creek fairway could produce a lot of water.

Cross section schematic (Source: Amelia Resources LLC)

Fortunately, for the downdip operators in LA-EAST,  a merged mega-3D seismic survey is available across most of the deep Tuscaloosa fields (Seismic Exchange Inc.).  We acquired most of these data in the early 90's while I was at Amoco. The 3D will be a key technological tool for success in that environment.  EOG and Devon have acquired most of the leases in this area.  It is for this reason that Marathon, in partnership with CGG and Fairfield, is acquiring a very large 3D survey (500 square miles) across their downdip acreage in LA-WEST.

A conventional core from West Feliciana Parish reveals a very cyclic sequence of chalk and chalky-marl facies.  The higher TOCs occur in the chalky-marl facies. Higher gamma ray also occurs with the increase in marl content.  On a regional basis, the LA-West region generally contains a higher percent of chalk facies.  This is likely due to the ancestral Mississippi River sourcing clastics into the LA-EAST region.  In 1992 while at Amoco, our research center preformed a geochemical study on the Tuscaloosa Trend oils (LA-EAST).  The Austin Chalk was confirmed to be a source rock.  It's very possible that the hydrocarbons in LA-WEST are sourced from older formations.  The gas/oil ratios in Masters Creek Field greatly contrast those in southern Avoyelles Parish.  Resistivities are also lower in LA-WEST compared to LA-EAST.  

SP and Gamma Ray

Gamma ray correlated to facies and rock type

This play across Louisiana and Mississippi will likely see contrasting results in the four sub-regions: LA-EAST Updip and Downdip, LA-WEST Updip and Downdip.  Each sub-region appears to have its "pros and cons".  Simple geology is great for drilling and producing.  Many like oil over gas.  Shallower and simpler is cheaper.  Pressure "is your friend" when producing low perm reservoirs, but it's not easy to drill through.  Marathon's first well is evidence to that.  Faulted structures with varying dip orientations are challenging for planning a horizontal drilling program.  An abundance of old vertical wells penetrating the Austin Chalk presents the opportunity to perform detailed log analysis with an understanding of spatial variability.  Thick high TOC with higher porosities is always the best.  And most importantly, the highest IRR's provide play longevity.

No comments:

Post a Comment