Abstract

The Upper Jurassic lime mudstones of the Lower Smackover Brown Dense Limestone constitute a major source rock in the U.S. Gulf Coast area, both onshore and offshore. Interest in the Brown Dense Limestone as a potential unconventional oil resource play intensified as oil prices climbed above $100/barrel in 2008. Because of its moderate subsea depth in the Southwest Arkansas / Northwest Louisiana area, many of the established unconventional resource players chose to focus their leasing and drilling efforts in that part of the trend, despite the fact that few wells had been drilled through the entirety of the Brown Dense Limestone in that area, and almost all of those wells that had been drilled through the entire formation had failed to encounter any substantive reservoir rock.

In March 2009, in the Tick Creek Field area of Union Parish, Louisiana, while drilling a Smackover wildcat well (#2 Exxon-Mobil, API # 17111256100000), the Weiser-Brown Operating Company unexpectedly encountered a strong influx of oil and gas from a geopressured reservoir within the Middle Brown Dense Limestone at a driller’s depth of 9142 feet, and was forced to set casing (without running open-hole logs) to 9100 feet to control the influx of oil and gas. Rumors that the well had flowed at rates exceeding 600 BOPD from the 40 feet of open hole interval below the casing shoe sparked an intense leasing program and the drilling of numerous vertical and lateral wellbores targeting the Brown Dense Limestone, mostly in the Southwest Arkansas / Northwest Louisiana area. Other wells drilled to test the Brown Dense Limestone as an unconventional oil resource were located in south and west-central Mississippi.

Ironically, almost all of the “unconventional” wells drilled between 2009 and 2015 were drilled in areas where there is virtually no porosity or permeability in the Brown Dense Limestone. The micritic facies typically has 1% to 4% porosity and nil permeability. In addition, in much of the Trend area the Middle and Lower Brown Dense Limestone intervals are highly laminated, with alternating thin layers of extremely hard and brittle, pure (“clean”) micrite and more ductile, argillaceous mudstone. These interlaminated “brittle/ductile” intervals are very difficult to fracture-stimulate, especially in a lateral wellbore; for this reason, frac half-heights of most stimulated Brown Dense laterals have rarely exceeded 75 feet. It appears the brittle laminae are intensely microfractured in certain areas of dip/slope change, creating several thin microfractured reservoirs that are confined between the more ductile argillaceous laminae above and below.

This relative lack of “storage”—and the lack of permeability to permit the flow of in-situ hydrocarbons to the wellbore—has yielded predictable (non-commercial) results, and the oil price crash of 2015 further diminished interest in the Southwest Arkansas / Northwest Louisiana Trend. Most of the wells that were completed in the Brown Dense Limestone during that six-year period are now abandoned, shut in or produce at rates of less than 20 BOPD. This includes the Weiser-Brown #2 Exxon-Mobil, the vertical well that had initially ignited the trend.

The companies that had targeted the Brown Dense Limestone as an unconventional resource had either intentionally or unintentionally chosen to concentrate their exploration efforts in the extremely “tight” micritic areas of the Trend. However, in other more favorable areas, thick reservoirs with fair to good porosity and permeability are known to exist within the Brown Dense Limestone. Vertical wells that have encountered such porous and permeable zones within the interval have flowed at rates in excess of 2,-300 BOPD in Mississippi (without fracture stimulation), and prolific producers of oil, gas and condensate have been completed in Brown Dense reservoirs elsewhere in the state and in neighboring Alabama. From a historical perspective, the first lateral wellbore intentionally drilled to target a Brown Dense Limestone reservoir was completed as an open hole oil producer in the Vossburg Field in southeast Mississippi in 2000, fully nine years before the Weiser-Brown #2 Exxon-Mobil well was drilled in Union Parish, Louisiana.

Favorable porosity development within the Brown Dense Limestone can be encountered within embedded siliciclastics, carbonate lenses, or macrofracture swarms. Thick porous and permeable sandstones have been deposited within the Brown Dense in an area where a large fluvial/deltaic system (attributed to the ancestral Mississippi River) contributed significant quantities of siliciclastics and organic matter into the anoxic, low-energy (stagnant) muds of the Brown Dense Sea.

The deposition of thick clastics within the Brown Dense Limestone often led to depositional loading and early salt movement, such that thick Brown Dense Limestone clastic depocenters are now encountered on the flanks of the salt structures formed by such depositional loading. Conversely, wells drilled atop such salt structures frequently encounter little or no clastics within the Brown Dense Limestone, because the crests of such salt structures are far removed from the thick, sand-rich depocenters located on their flanks. Because most Smackover test wells drilled to date have targeted only the crest of such structures, many highly-prospective, sand-rich areas of the Brown Dense Limestone Trend remain untested on the flanks of those structures.

Where the mouth of the ancestral Mississippi River flowed into the stagnant waters of the Brown Dense Sea, the absence of any substantive Norphlet Sand in that area meant that typically less than five feet of compacted Basal Norphlet mud (shale) separated the marine waters of the Brown Dense Sea from the underlying thick salt playa evaporites of the Louann Salt. Seismic data and well control indicate several episodes of storm surges or flooding events occurred during the earliest deposition of the Brown Dense Limestone. During those short-lived but very consequential episodes, the current energy of the ancestral Mississippi River was much higher, causing its current(s) to erode downward through the thin Basal Norphlet shale and exposing the underlying salt to substantial erosion.

In the deepest eroded areas, it appears all of the Louann Salt (as much as 2000 feet of bedded evaporites) has been dissolved and removed in a large scour. These eroded areas were rapidly and completely backfilled with thick organic-rich muds, and it appears that mounds of coarse-grained sediment were deposited in the central areas of such erosional features. The dissolution and removal of the salt, and its subsequent backfilling with thick Brown Dense Limestone muds and sands, has resulted in an aggregate “local” gross thickness of Brown Dense Limestone sediments that often exceeds 2500 feet.

The dissolution and removal of the Louann Salt in such areas also initiated early movement of the adjacent, uneroded salt in response to the lateral forces associated with the backfilling and depositional loading of the eroded areas.

Shallower water and more favorable, less hostile conditions led to the development of thrombolites and peloidal / pelletal lenses in certain Brown Dense Limestone areas, especially in South Alabama and Northeast Texas. These porous carbonate facies have produced significant volumes of hydrocarbons in certain Trend areas, especially South Alabama.

Macrofracture swarms (drag/shear breccias) charged with hydrocarbons have been encountered on the downthrown side of large normal faults that may trend for miles and favorably juxtapose the downthrown Brown Dense Limestone against the Louann Salt. These macrofractures swarms can be geopressured and represent an optimal target for lateral drilling.

To date, none of these favorable areas of porosity development within the Brown Dense Limestone have been targeted by those companies interested in its unconventional resource potential. The existence of these prolific “conventional,” stratigraphically-trapped reservoirs—encased entirely within one of the most prolific source rocks of the entire U.S. Gulf Coast area—sets up a “hybrid” unconventional resource play, where lateral drilling and fracture stimulation can efficiently access and drain the potentially large volumes of hydrocarbons that are trapped within the numerous porous and permeable reservoirs of the Brown Dense Limestone.