Atchafalaya basin

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History of the basin

Located in south-central Louisiana, the Atchafalaya basin is a constant growing basin that covers 140 miles and is the largest wetland in the United States. This basin is bordered by the Mississippi River to the north, Bayou Shaffer moving down South, and a levee system stretches from Simmesport, LA to the Gulf of Mexico. This congregation of swamps and deltas are located at the point where the Atchafalaya river and the Gulf of Mexico intersect. The area is home to about 60 species of reptiles and amphibians, over 270 species of birds, and almost 100 species of aquatic life.[1] The basin also consists of 70% heavily wooded forests and 30% swamp and open waters, and with the constantly growing delta system, flooding and land loss is very prominent in this area (fig. 1). Between the years of 1932 and 1990, the total wetland loss in the Atchafalaya area was around 3,760 acres, with an average loss of 87 acres per year between the years 1974 and 1990[2]. The discovery of oil and gas in the 1920s as well as numerous environmental forces, such as the widespread flooding, ultimately helped form the existing environment surrounding the basin. Recently, one of the main contributors to the widespread flooding in the Atchafalaya area over the years, the Mississippi river has changed its course and dumping a lot less water into the basin.

Primary Geological Risks and Uncertainties

Fig 1. Wetland Loss in basin

Wetland loss in the basin is dependent on the environment and what is going on around it. The constant human activity (the pipeline systems stored below for example), erosion from storms and hurricanes, and other natural occurrences, like deltas and levees, all contribute to the overall disruption of the wetland. The history of the delta growth is constant and this means land loss will not slow down soon. In the next 50 years, approximately 67,000 acres in surrounding areas will be dominated by the surrounding delta systems[2]. This delta growth and land loss lead to long-term overall stability concerns within the basin. To fix this problem, some nature conservation groups are working on ways to minimize delta growth and increase the sediment being delivered to the basin, which has been restricted by a wide range of environmental factors. Another thing that is a huge threat to the environment is the oil and natural gas exploration that has been going on for decades. Though the rate of exploration has decreased with time, the pipeline systems promote a faster rate of environment fragmentation/loss and pollution. The old pipeline systems help fragmentation and act as a conduit for invasive species, pollution, and saltwater infiltration. [1]

Petroleum Elements

Source Rock and Reservoir

Deep Miocene and Pleistocene formations composed of mainly sandstones make up the source rock of this basin. These rocks are composed of a majority of quartz and feldspars. The quartz-rich sandstones with high permeability make for the best reservoir quality in this area. [3]. The basin is home to different many different back swamp clays. With the addition of the high pressures and temperatures, the highly organic Pleistocene clays of the sediment deposits in the basin, as well as the good rate of sinkage and sedimentation, allow for adequate maturation of hydrocarbons in the basin. Mudstone works as the reservoir rock in this basin; its high permeability, porosity, and carbon-rich sweet spots for hydrocarbon production make it an excellent source rock.

Fig 2: Salt dome formation

Traps and Seals

There are different Traps in the Atchafalaya, but the majority structural anticlinal. The most common trap being a salt dome, which is caused by a structural anticlinal trap and acts as a Hydrocarbon seal. A salt dome occurs when a section of salt rises upwards, creating an anticline deformation in the rock [4](fig. 2). These kinds of traps are abundant in Louisiana, because of the low cost of maintenance and overall convenience compared to the other methods. These deformations create traps with the shale for underground oil and gas storage that we obtain with vertical drilling methods.

Migration

Hydrocarbons formed from marine organisms along with the highly permeable source rock and reservoir, cause heavy, constant migration into many different traps, with the main trap being salt domes. With the high permeability and migration, it makes drilling in this area slightly difficult because the substances below can move and relocate.

Future Petroleum Potential

For over 60 years, the Atchafalaya basin has been home to many oil and gas fields. Production in this area surrounded by levees has created thousands of jobs for the state of Louisiana. As of now, there are 46 wells in the basin, but 42 of them were done with production around 40 years ago. [5] So as of now, it seems like we have drilled all with can drill with the technology we have today. With the demand for oil being like it is, deeper and more extensive exploration should be necessary. With that being said, of all the petroleum exploration fields in the Atchafalaya, only a few have adequately tested the deep oil production below. With more time and technology, the Atchafalaya basin can be explored and tested more thoroughly for further production. [6] One thing that could limit the amount of further production is the aspect of space below the surface; the Atchafalaya basin is riddled with a spiderweb of oil and gas lines below. The pipelines cause major environmental concerns as well as social concerns with the residents of the basin area.

Engineering Aspects

Fig 3: Louisiana pipeline system

The pipeline system in Louisiana looks like a huge spider web (fig. 3). With the newest edition of the Bayou Bridge Pipeline in 2016, Louisiana added another 163 miles to its pipeline system. With as much as Louisiana has riding on the oil and gas industry, it is understandable to have the number of pipelines they do. But, with this mass number of pipelines comes environmental concerns and backlash from some residents in this area that depend on the land for their livelihoods. [7]

Further Readings

The Atchafalaya basin CWPPRA

Geol. Investigation of the Atchafalaya Basin

Salt Dome Importance to Lousiana

Mississippi River Changing Course

References