fluid migration — Blog — McLindon Geosciences, LLC

The sedimentary layers underlying south Louisiana are saturated with fluid, and those fluids are in constant motion. Saltwater makes up the overwhelming majority of subsurface fluids. Most sediment deposition takes place in shallow seas along the delta coastline. Sediments have their maximum fluid content at the time of deposition. Flocculated clays can be up to 80% water and sands can have porosities approaching 40%. Compaction begins immediately after a sedimentary layer is buried by a younger layer. The weight of the overburden squeezes the underlying sediment. This increases the grain-to-grain contact of the mineral matrix, making the pore spaces smaller and forcing the pore fluid out. Compaction progresses with depth of burial as long as fluids can be expelled. In deep marine (or abyssal) sediments sands are thinner and less continuous. The free flow of fluids is inhibited in these sedimentary layers. The inability to expel pore fluids prevents continued compaction, and the fluids become highly pressured (or geopressured).

William Galloway of UT Austin and the Texas Bureau of Economic Geology defined three major regimes of subsurface fluid flow. The abyssal or thermobaric regime consists of highly pressured marine sediments. Hot, high pressured fluids are forced out primarily along faults. The movement of pressured fluids along fault planes is an integral part of the mechanics of fault slip. The elisian or compactional regime lies above the top of geopressure. Interstitial fluids expelled from within the sediments in this regime follow natural pathways of fluid flow. These are usually through the porous and permeable sand layers, but fluids can also move vertically up faults. The fluids moving up from the abyssal regime may also enter the pathways in the elisian regime. These fluids are not only hot and pressured, they may be hypersaline and carrying oil and natural gas cooked out of organic-rich source rocks. Most of the oil and gas produced in south Louisiana comes from accumulations in sand layers in the elisian regime just above the top of geopressure. The shallowest sedimentary layers may be exposed at the surface further inland. These layers make up the meteoric or infiltration regime. The saltwater in the pores of sands in this regime has been flushed out by rainwater that moves by artesian flow through the aquifers.

Many faults in south Louisiana extend into the meteoric regime. Segments of the Baton Rouge fault system commonly form boundaries between freshwater and saltwater aquifers. Hypersaline fluids can also migrate from the faults into the freshwater aquifers. Ronald Stoessell and Lesley Prochaska at the University of New Orleans studied brackish water found in wells in St. Tammany Parish. They determined that the brackish water was “derived from deep migrating formation fluids from dissolved halite migrated up fault planes”. Gerald Kuecher and Harry Roberts at LSU studied faults in Terrebonne Parish. They also found evidence for the migration of saline fluids up those fault planes. Their diagram of fluid flow up a fault plane into a reservoir sand illustrates the fractured nature of a fault plane. It is often envisioned that faults create a zone of rubble along the plane that is both porous and permeable. Mark Rowan’s block diagram shows how faults are a part of a linked tectonic system. Along with salt (in red) and salt welds, which are the remnant pathways of salt movement, faults provide essential conduits for fluid flow. Arrows of fluid movement are overlain on Rowan’s original diagram.

The Montegut fault is in the area studied by Kuecher and Roberts. The fault plane is displayed as a colored surface extending from a depth of 15,000 feet along the dark blue band up to the visible trace of the fault at the surface. The Montegut fault is a part of the linked tectonic system across coastal Louisiana called the Terrebonne Trough. Faults along the northern boundary of the trough are dipping to the south. Faults along the southern boundary are dipping to the north. Sedimentary layers are thicker within the bounds of the trough. The surface expression on most of these faults is visible on the USGS Land Area Change Map (Couvillion et al, 2017). Fault traces and the distributary channel network have been overlain on the original map. Hot spots of land loss shown in red and orange tend to lie along the downthrown sides of the faults at the points where the distributary channel networks branch out. Each of these branching channel networks represents the architecture of one of the Holocene deltas of the Mississippi River. It appears that the Mississippi River moved back and forth across the Terrebonne trough constantly seeking out maximum accommodation capacity for its sedimentary load. The Montegut fault was an integral part of the Bayou Terrebonne delta system, which was active between about 800 and 500 years ago.

A three-dimensional diagram shows the contours of the fault plane dipping to the south. The surface trace of the fault crosses the branching network of the former distributary channels of the delta system. The natural levees of these distributary channels provide the elevation necessary for human inhabitation in this area.

At a depth of about 10,000 feet below the surface the Montegut fault intersects a Miocene sand layer. The New Orleans Geological Society (NOGS) identified this sand as the “Big 2 Sand” for its association with a microscopic foraminifera called Bigerina 2. Foraminfera found in sediments and recovered from the cuttings of a drilling well are the most common method for age-dating Miocene sediments. The NOGS subsurface structure map on the Big 2 Sand shows the concentric contours of the anticlinal structure of Lirette Field in Terrebonne Parish. The northern edge of the anticline is crossed by the trace of the Montegut fault. Gas reservoirs accumulated in the Big 2 sand across the top of the anticline are shown in red. A study of the subsurface controls on historical subsidence rates by the USGS (Morton et al, 2002) noted that the discovery of Lirette field was aided by the detection of gas seeps at the surface. The study found that “The seeps indicate that before commercial development began, fluids were migrating vertically along deep fault planes that intersect the subsurface gas reservoirs”.

Lirette Field was studied by Leigh Anne Flournoy and Ray Ferrell at LSU. Several whole cores of reservoir sands taking during drilling operations allowed them to do detailed examination of the mineral matrix of the sand layers. They determined that a sequence of diagenetic events had occurred over time in association with fluid movement through the sands. These included both the deposition of minerals that were precipitated from fluids and the dissolution of other minerals by fluid flow. Flournoy and Ferrell identified likely routes of fluid movement from the faults into the reservoir sand layers. A profile across the Montegut fault shows the patterns of fluid movement. The movement of hotter fluids migrating up the fault plane into the sands is also evidenced by the displacement of the 200° F isotherm along the fault.

The surface trace of the Montegut fault was documented in the study “Synthesis of Fault Traces in SE Louisiana Relative to Infrastructure” (Culpepper et al, 2019). The surface trace determined by mapping the fault with 3-D seismic data coincided with the surface expression of the fault determined by shallow borings by Gagliano et al, 2003. A profile of borings across the fault shows thicker accumulations of peat on the downthrown side of the fault relative to the upthrown side. The accumulation of plant material that makes up a peat layer is evidence of the marsh’s attempt to keep up with subsidence by organic growth. This profile is evidence of subsidence caused by slow slip movement on the Montegut fault over the centuries since the Terrebonne Bayou delta system was abandoned. The elevation change across the fault at the surface causes a sharp transition from emergent marsh to open water. This coincides with the area of recent land loss on the USGS Land Area Change Map, which may evidence a recent fault slip event.

Kuecher and Roberts examined soil salinities across southeastern Louisiana. A colored recreation of their map shows a distinct anomaly in soil salinity (measured as total dissolved solids in parts per thousand) in the vicinity of the Monetgut fault. They determined that this anomaly could not be explained by traditional means of saltwater encroachment. A photo from Gagliano et al, 2003 shows the proximity of an area of dead cypress trees to the Montegut fault. It may be possible that the increased soil salinities along the trend of the fault may have played a role in the death of the trees.

The Montegut fault exhibits evidence of continual episodic movement since the middle Miocene. There is also evidence of fluid migration associated with fault movement from that time until the present. Understanding the relationships between fault movement, fluid migration, subsidence and wetlands loss will be essential to the ultimate success of long-term sustainability planning in coastal Louisiana.

REFERENCES

Couvillion, B.R., Beck, H., Schoolmaster, D., and Fischer, M., 2017, Land area change in coastal Louisiana 1932 to 2016, U.S. Geological Survey Scientific Investigations Map 3381, 16 p. pamphlet, doi.org/10.3133/sim3381.

Culpepper, D.B., McDade, E. C., Dawers, N. H., Kulp, M. A., Zhang, R., 2019, Synthesis of Fault Traces in SE Louisiana Relative to Infrastructure, TranSET Project No. 17GTLSU12

Flournoy, A.L. and Ferrell, R.E., 1980, Geopressure and diagenetic modifications of porosity in the Lirette Field area, Terrebonne Parish, Louisiana, GCAGS Transactions, v 30, p 341-344

Gagliano, S.M., Kemp III, E. B., Wicker, K. M., Wiltenmuth, K. S., 2003. Active Geological Faults and Land Change in Southeastern Louisiana. Prepared for U.S. Army Corps of Engineers, New Orleans District, Contract No. DACW 29-00-C-0034.

Galloway, W. E., 1982, Epigenetic Zonation and Fluid Flow History of Uranium-Bearing Fluvial Aquifer Systems, South Texas Uranium Province: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 119, 31 p.

Kuecher, G.J., Roberts, H.H., Thompson, M.D. and Matthews, L., 2001, Evidence for Active Growth Faulting in the Terrebonne Delta Plain, South Louisiana: Implications for Wetland Loss and the Vertical Migration of Petroleum., Environmental Geosciences, v. 8, p. 77‐94

Morton, R.A., Buster, N.A., Krohn, M.D., 2002, Subsurface Controls on Historical Subsidence Rates and Associated Wetland Loss in Southcentral Louisiana, GCAGS Transcations, v. 52, p. 767-778

New Orleans Geological Society, Oil and Gas Fields of Southeast Louisiana Volume 1, 1963

Stoessell, R. K. & Prochaska, L., 2005, Chemical Evidence for Migration of Deep Formation Fluids into Shallow Aquifers in South Louisiana, GCAGS Transactions, v. 54, p. 794-808