LA LT Discovery
Port Hudson Field
ONE OF THE MOST PROLIFIC (AND SENSITIVE) RESERVOIRS IN THE GULF COAST
For a Special Report on the Middle Tuscaloosa Marine Shale, also known as the "TMS", please click here.
The Tuscaloosa Formation outcrops in the northwest area of Alabama, thus deriving its name from the city of Tuscaloosa (itself named after the Indian word - and the name of the great Indian chief - meaning "Black Warrior"). However, the formation that outcrops is but a thin updip representation of a much greater sedimentary wedge that, to the south and west, is informally subdivided within the subsurface into the Upper, Middle, and Lower Tuscaloosa "Formations". Specifically, the Lower Tuscaloosa represents the oldest Upper Cretaceous fluvial-deltaic complex encountered in the Alabama / Mississippi / Louisiana area, and is interpreted by most geologists to either pinch out, or be entirely eroded, within its updip subsurface perimeter in north and east Mississippi - and thus is not distinctly represented in the northwest Alabama outcrop facies.
The Lower Tuscaloosa is further (and informally) subdivided into three distinct units. From bottom (oldest) to top (youngest), they are as follows:
"Massive" Sand - this is the fairly ubiquitous basal transgressive sand encountered atop the major unconformity at the top of the Lower Cretaceous. Sand thickness can range from 30 feet to well over 300 feet for individual sand lobes in the expanded Massive Sand facies encountered in the Deep Lower Tuscaloosa Gas Play of south-central Louisiana. Because of its position atop the LK Unconformity, the lowermost part of the Massive Sand typically contains large amounts of gravel, consisting predominantly of eroded and redistributed Devonian chert from the southern Tennessee area.
From an economic standpoint, the Massive Sand is by far the most important unit of the Tuscaloosa Formation, with many examples of large reservoirs such as those found at Baxterville Field (> 250 million barrels oil) in southwest Mississippi, and Port Hudson Field (> 760 billion cubic feet of gas) in south-central Louisiana. Because the Massive Sand is distributed over such a large geographic area, its geomorphology and lithology suggests a general depositional setting similar to that of modern-day braided streams, such as the Mackenzie River Delta of northern Alaska. The sheet-like nature of the sandstone also provides for strong water drive in most Massive Sand reservoirs. However, the same sheet-like character restricts the Massive Sand reservoirs to structural and fault closures (i.e., not stratigraphic pinchouts - except in certain rare circumstances). The sandstone has a "salt & pepper" appearance and consists predominantly of coarse to medium-sized quartz sand grains with very small amounts of kaolinite and chlorite; for this reason, clay constituents typically do not impact wellbore producibility (except, it appears, in the Adams County Uplift / LaSalle Arch areas, where the clay content sporadically climbs as high as 10% total sediment volume in some wells). Average porosity is 17% with permeability ranging from nil in the tight cherty facies to two darcies in the best coarse-grained reservoirs. Recoveries can top 500 barrels per acre-foot in Massive Sand oil reservoirs with strong water drive, such as within the Baxterville Field complex. Click here to view a Type Log for the Lower Tuscaloosa at Baxterville Field. The Massive Sand has also proven its capability to flow large volumes of natural gas; for example, several wells recently completed in Judge Digby Field, in Pointe Coupee Parish, Louisiana, has sustained monthly production rates of 1.8 billion cubic feet of gas ("BCF") per month, or 60 million cubic feet of gas per day. That is a world-class gas completion by anyone's standards.
"A" and "B" (Upper "Stringer") Sands - encountered just above the Massive Sand, the Lower Tuscaloosa A and B Sands are much more lenticular in nature. The lower of the two sands - the B Sand - represents a classic fluvial-deltaic system in the southwest Mississippi area, as does its younger counterpart, the A Sand. In extreme southwest Mississippi and the Florida Parishes area of Louisiana, both the A and B Sands transition from fluvial to intertidal/deltaic in geomorphology, similar to that of the modern Mobile Bay, with some sands exhibiting low-energy marine (intertidal) reworking. In some areas, the A Sand is observed to have "downcut" (eroded into) the top of the underlying B Sand; this observation is believed to be linked with the rarity of the B Sand oil reservoirs in those particular areas. For that and other reasons, the A Sand is the more prolific and widespread reservoir, with important accumulations in the Little Creek, Smithdale, and Liberty Field areas of southwest Mississippi, as well as the Greensburg Field area of central St. Helena Parish, Louisiana, to name a few. Click here to view an example of an "A" Sand producer from Millbrook Field in Wilkinson County, Mississippi. Unlike the Massive Sand, the Lower Tuscaloosa A and B Sands typically contain 3% to 9% chlorite (giving the sands their characteristically greenish appearance, and "ashy" feel and description). Kaolinite is also more abundant, especially with increasing depth. The pore-lining platelets of the chlorite greatly increase surface area and capillary pressure, causing these Lower Tuscaloosa reservoirs to typically have a very low hydrocarbon-productive log resistivity cutoff. This geologist has observed water-free oil and gas production from A Sand reservoirs that had measured log resistivities as low as 0.3 ohm-meters. The average induction resistivity for a productive Lower Tuscaloosa A or B Sand is 0.5 to 0.8 ohm-meters (see the productive example that follows this discussion, below). Average porosity is 22%, but frequently tops 28%; permeability ranges from nil in very tight siltstones to well over two darcies in the best reservoir rocks. The reservoir drive for these lenticular reservoirs is predominantly depletion-drive, except for the surprisingly well-connected fluvial point bar and channel complexes within the greater Lincoln County, Mississippi area (including Mallalieu, Smithdale, and Little Creek Fields). The more significant clay content of the A and B Sand reservoirs makes them much more sensitive to drilling and completion fluids, as well as acid and fracture stimulation treatments; for example, common hydrochloric acid attacks the chlorite platelets, causing partial platelet dissolution and the formation of a gel-like substance that dramatically reduces near-wellbore permeability. Overbalanced or underbalanced perforating can dislodge the kaolinite "bookends", causing severe pore throat blockage associated with the "migrating fines". Caution and experience is invaluable when attempting to drill, cement, and test these very sensitive sandstone reservoirs.
"Pilot" Sand - located at the top of the Lower Tuscaloosa Formation, the "Pilot Sand" was so named because wellsite geologists, desiring to cut a diamond ("whole") core through the underlying A Sand, became accustomed to observing a drilling "break" at the top of the Lower Tuscaloosa that was attributable to a thin, often tight and silty sandstone that had been previously unnamed because of its lack of commercial reservoir porosity and permeability. This thin sand became known as the "Pilot Sand" because, in essence, it "piloted" (guided) the wellsite geologist toward the more important goal of picking the A Sand core point at what was pretty much a consistent depth below the Pilot Sand break. Except in very rare circumstances (such as within the Hub Gas Field in Marion County, Mississippi), the Pilot Sand - for the reasons noted above - harbors little commercial potential for oil and gas production.
The Deep Lower Tuscaloosa Trend is only partially developed. Most of the existing production has been established in Pointe Coupee and East and West Baton Rouge Parishes. With the exception of Port Hudson Field (located primarily in East Baton Rouge Parish), the reservoirs are comprised of "3-way" and "4-way" fault closures that form on the downthrown side of a series of "stair-stepping" down-to-the-coast normal faults (Port Hudson comprising (to date) the lone salt-cored producing structure). Click here to view a subsurface structure map of Port Hudson Field, contoured on the Top Lower Tuscaloosa / Base TMS "Bain" Marker. Reservoir depth generally ranges from 15,000' on the northern perimeter of the Trend to 24,000' on the southern perimeter. Prospects located within this Trend form an integral part of Vision's Exploration Program; click here to view a log montage of several productive Deep Lower Tuscaloosa wells.
Steve Walkinshaw, President, Vision Exploration
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