Jeffrey A. Nunn

Episodic fluid expulsion from geopressured sediments

Numerical simulation of fluid explusion from geopressured sediments using a one-dimensional finite difference model shows that large volumes of fluid can be transported through a fractured seal during short-lived (<100 year) expulsion events. In this model, abnormal fluid pressures are maintained by an impermeable seal. When the fluid pressure exceeds 85% of the lithostatic pressure, a vertical fracture opens in the seal, providing a permeable pathway from the geopressured sediments into the overlying section. Fluid pressure in the seal decreases as fluid flows through the fracture, closing the fracture as sediments in the seal compact. Once the fracture closes, the seal remains impermeable until fluid flowing from deeper in the section increases fluid pressure to the fracture criteria, a process that takes 10 000–500 000 years. Fractures emplaced in relatively incompressible seals, such as compacted and/or cemented sediments, remain open for 20–50 years; fluid fluxes through the seal integrated over the life of these events are 1−5×107 kg/m2, and temperatures in the overlying sediments are increased by 5–20°C. These temperature anomalies decay to less than half their original value in 300 years, suggesting that the thermal anomalies observed in wells that are attributed to fluid expulsion formed in response to a very recent or currently active event.

Numerical simulations of brine migration by topographically driven recharge

The migration of abnormally warm, saline water through the Appalachian basin and North American midcontinent in Paleozoic time has been inferred from fluid inclusion studies, remagnetizations, and widespread potassic alteration. A time-dependent numerical model of fluid, heat and solute transport is used to evaluate the viability of topographically driven recharge as a mechanism for brine migration. The model represents a wedge-shaped sedimentary basin 400 km long by 6 km deep (maximum) with a basal aquifer 500 to 750 m thick overlain by a homogeneous aquitard. Temperature predicted by model simulations is found to be inconsistent with constraints inferred from fluid inclusion studies, unless average heat flow values greater than about 100 mW/m2 are used. Model simulations also lead to predictions of low heat flow and subsurface temperature in recharge zones that are generally not observed in modern orogenic zones. The initial solute content of pore waters in the model basin is flushed out by fresh water entering in the recharge zone before fluid velocities high enough to produce significant warming of the discharge zone can develop. Model simulations with source terms reveal that basin sediments can provide enough solute to maintain hot, hypersaline brine migration for about 1 m.y., at most. High fluid velocity in the basal aquifer is required to carry heat to the basin margins, but the higher the fluid velocity, the more quickly the basins supply of solute is exhausted. Consideration of these constraints implies that topographically driven recharge may be an effective mechanism to explain regional brine migration only if flow is focused from regional scale recharge zones into more spatially restricted discharge zones.

Expulsion of abnormally pressured fluids along faults

Numerical simulations of fluid flow and heat transport in the South Eugene Island minibasin, offshore Louisiana, show that expulsion of geopressured fluids along faults can produce temperature and pressure anomalies similar to those observed in the area. In the simulations, abnormally pressured fluid moves along the fault through a fracture network. A thermal anomaly forms adjacent to the fault, while a larger fluid pressure anomaly extends into sediments on either side. Results from constant fault permeability simulations indicate that (1) geopressured sediments must be relatively permeable (5 × 10−17 m2) for expulsion to occur, (2) the size of thermal anomalies depend on the depth to which the fault is hydraulically open, and (3) fluid is vertically transported into shallow sediments when fault permeability is high, while lateral transport along deeper sands dominates when fault permeability is low. Excess fluid pressure in abnormally pressured sediments drops to half its original value throughout much of the minibasin after 10,000 years of expulsion; the associated thermal anomaly is also larger than observations, suggesting expulsion is not continuous. Variable fault permeability simulations, in which compaction of fault zone sediments closes the fracture network, indicate that fault permeability decreases by 1–2 orders of magnitude 1–200 years after expulsion begins. Thermal and baric anomalies from variable permeability simulations are smaller than from constant permeability simulations and are more consistent with available data. Faults must remain permeable for 20–30 years to produce thermal and baric anomalies similar to those observed in the area.

Mechanisms driving groundwater flow near salt domes

Groundwater flow near salt domes is complex because groundwater is subject to a variety of driving forces including the release of geopressured fluids, large lateral density gradients, and regional hydraulic head gradients. The complexity of this environment is born out by recent geochemical and geophysical observations that indicate the occurrence of upward groundwater flow near some salt domes. In order to evaluate the relative importance of different mechanisms driving groundwater flow near salt domes, the authors have developed a numerical model that couples groundwater flow, heat transport, and transport of dissolved salt, and accounts for salt diapirism. The calculations indicate that upward groundwater flow can occur as the result of thermal convection when the regional background salinity is greater than 15 weight percent, a value typical of many areas of the south Louisiana salt dome province. For lower background salinities, dissolution causes salt-laden groundwater near the dome to sink, leading to depressed isotherms. While the release of geopressured fluids is difficult to quantify, it remains a likely mechanism for driving upward groundwater flow near some salt domes.

Buoyancy‐driven propagation of isolated fluid‐filled fractures: Implications for fluid transport in Gulf of Mexico geopressured sediments

A large portion of the sediments within the northern Gulf of Mexico contain pore fluid pressures in excess of hydrostatic. Development of geopressure is generally attributed to compaction disequilibrium caused by rapid deposition of low-permeability sediments in the Miocene and Plio-Pleistocene. Numerous studies have examined the formation of overpressures and/or expulsion of geopressured fluids into overlying hydropressured strata. However, very little attention has been given to fluid flow within the geopressured zone itself. Movement of oils from Cretaceous or older source rocks into Plio-Pleistocene reservoirs in the Gulf Basin requires as much as 10 km of vertical migration in a few million years. Precipitation of cements in some geopressured sediments also implies large-scale fluid flow. New evidence from a deep well in the Eugene Island area, offshore Louisiana, indicates that geopressured sediments are mechanically very weak with a Poissons ratio greater than 0.4 and a shear modulus or rigidity less than 1 GPa. In addition, large-scale fluid flow either through interconnected pores or fractures is not occurring in this location, at least at present. An alternative hypothesis is that upward fluid transport in geopressured sediments is caused by buoyancy-driven propagation of isolated fluid-filled fractures. Using linear fracture mechanics, I show that vertical fractures with lengths of a few meters can propagate at velocities of 1000 m/yr. Mass flux rates (∼100 kg/m2/yr) are significant assuming a mechanism for formation of fluid-filled fractures exists, such as hydrofracturing when fluid pressures exceeded the minimum confining stress. Fracture propagation velocity and mass flux rate are strongly dependent on the shear modulus of geopressured sediments.

Thermal Subsidence and Generation of Hydrocarbons in Michigan Basin

Temperature histories for selected stratigraphic horizons in the Michigan basin are computed from three-dimensional continually filled subsidence models. Mechanical evolution of the Michigan basin is modeled as flexure of the lithosphere caused by thermal contraction. Results are compatible with the subsidence record of the sediments and free-air gravity anomalies. Paleotemperature is determined from excess temperature due to the thermal anomaly plus burial temperature predicted from subsidence curves. For an equilibrium temperature gradient of 22°C/km (1.2°F/100 ft), surface temperature of 20°C (68°F), and an equilibrium surface heat flow of 1.1 HFU, excess temperature, paleotemperature, and surface heat flow do not exceed 15°C (27°F), 110 76;C (230°F), and 2.5 HFU, respectively. These estimates are consistent with upper limits set by paleomagnetic studies. The low value for excess temperature is caused by concentration of the thermal anomaly below 15 km (9 mi), in agreement with gravity results. The great depth of the thermal anomaly can explain the lack of evidence for an initial heating event prior to subsidence. Once the thermal history of the sediments is specified, the oil potential of the basin can be determined from laboratory-derived kinetic equations for degradation of kerogen to petroleum. For the Michigan basin, predicted temperature conditions are sufficient for source rock (> 25%) conversion of type I and type II kerogen only in Ordovician and older rocks in the southern peninsula of Michigan. By this model, petroleum found in rocks younger than Ordovician would have had to migrate upward from the older rocks. Geochemical studies of Dundee (Devonian) and Trenton (Ordovician) crude oils in Michigan are compatible with this interpretation. Niagaran (Silurian) crude oils appear to come from a different source than the Dundee and Trenton oils. If their source is Silurian rocks on the flanks of the basin, either the scheme used to calculate maturity is not applicable to these oils or the temperature of the source rocks was significantly higher in the past than today. More likely, the model underestimates the depth of burial of Silurian source rocks near the center of the basin, especially in local regions of downfaulting.

Kilometer-scale upward migration of hydrocarbons in geopressured sediments by buoyancy-driven propagation of methane-filled fractures

Several lines of evidence support kilometer-scale upward migration of fluids in the Gulf of Mexico Basin: discharge of hypersaline brines at the sea floor; long-term, natural hydrocarbon seeps and microseeps; gas chimneys; lead-zinc mineralization in salt dome cap rocks; and allochthonous brines in Cenozoic sediments. We explore the hypothesis that upward fluid transport in geopressured sediments is caused by buoyancy-driven propagation of isolated methane-filled fractures. In other words, instead of fluid migrating along a fixed network of interconnected pores or fractures, fluid enclosed within an isolated fracture is transported upward by hydrofracturing the mechanically weak geopressured sediments. Thus, the fluid-filled fracture propagates upward through the sediments. Hydrofracture is driven by the pressure difference (buoyancy) between the enclosed methane and the surrounding sediments. Our results show that methane-filled fractures with half-lengths of a few meters should propagate upward through geopressured sediments with velocities of hundreds of meters per year or higher. As methane-filled fractures increase in volume and decrease in density with decreasing confining pressure, they develop the potential to entrain and transport more than 1000 kg/m3 of oil or brine. Methane-filled fractures should propagate to the surface unless they are trapped beneath a layer that has high fracture toughness, such as salt, or are absorbed when they intersect a permeable (>10 md) sand layer.

Free thermohaline convection beneath allochthonous salt sheets: An agent for salt dissolution and fluid flow in Gulf Coast sediments

Basinward migration of Jurassic salt in the U.S. Gulf of Mexico has resulted in the emplacement of large allochthonous salt sheets into shallow Miocene to Holocene sediments. Although comparatively little direct information is available on the environment below these salt bodies, it is reasonable to suppose that the formation of dense brines by dissolution of the base of these sheets may induce free thermohaline pore fluid convection within the sediments below. We derived equations which make it possible to quantitatively estimate rates of dissolution of these subsurface salt structures. From these calculations and by geologically realistic numerical simulations it can be shown that free convection beneath allochthonous salt sheets has the potential for being a significant mechanism for both salt dissolution and mass transport, even if the underlying sediments have permeabilities as low as 10−17 m2 (0.01 mD). The calculated maximum Darcy fluxes and rates of salt dissolution rapidly increase with sediment permeability. When the vertical permeability of the underlying sediment is 10−17 m2 (0.01 mD), salt is dissolved from the base of the sheet at an average rate of 3–5 m m.y.−1 The corresponding fluid velocities are such that over a 10 m.y. period the integrated fluid flux in the underlying sediments would be ∼104 m3 m−2. By comparison, integrated fluid flux for compactive expulsion is <103 m3 m−2. Thus, for the offshore sediments of the Gulf of Mexico, thermohaline convection beneath an allochthonous salt sheet is a significant driving mechanism for fluid flow with potentially important implications for heat and mass transport, diagenesis, and salt tectonics.

The Framework of Hydrocarbon Generation and Migration, Gulf of Mexico Continental Slope

ABSTRACT The occurrence of large volumes of crude oil and thermogenic gas in Plio-Pleistocene reservoirs and Holocene seeps of the Gulf of Mexico slope argues that the process of hydrocarbon generation and migration continues at the present time. Calculated thermal maturity models based on deep seismic stratigraphy indicate that Cretaceous and possibly Early Tertiary sediments are presently within the thermal maturity range for generation and expulsion of crude oil. The Cretaceous and Early Tertiary sediments are suggested as possible source rocks because younger sediments are thermally immature for crude oil generation, and older sediments are thermally overmature for crude oil preservation. The calculated thermal maturity profile for the slope indicates that some generation of crude oil could occur as shallow as 6 km, and that liquid hydrocarbons could be preserved as deep as 9 km. Thermogenic gas is stable at even greater depths. Migration with a strong vertical component must be invoked to explain thermogenic hydrocarbons in shallow reservoirs and seeps of the slope. The great depth at which liquid hydrocarbons and dry gas can be preserved in reservoirs of the Gulf of Mexico Salt Basin indicates that huge volumes of prospective section remain essentially unexplored.

Subsurface seepage of seawater across a barrier: A source of water and salt to peripheral salt basins

Many of the great Phanerozoic salt basins share a common paleogeography, in which a deep lake or sea is separated from the ocean by a narrow barrier. Examples include the Aptian evaporites of the South Atlantic rift basins and the Messinian salt deposits of the Mediterranean Sea and the North Caspian depression. Marine transgressions over the barrier have been proposed as the origin for saline conditions in these basins. We test an alternate hypothesis, that subsurface seepage of seawater through the barrier was a significant source of water, influencing both water level and water composition. Our model permits flux of water and salt into and out of the basin via (1) seepage through the barrier; (2) evaporation from the water surface; (3) rainfall and/or rivers; and (4) incorporation into basin sediments. Our investigation focuses on three settings, a modern analog in the Gregory Rift of northern Kenya, the Messinian Mediterranean basin, and the Early Cretaceous Aptian salt basins of the South Atlantic. We test our model through a numerical simulation, applying reasonable values for the dimensions of the barrier, differences in water level, and hydraulic conductivities to calculate flow rates through the barrier. Modeling results indicate that flux of seawater through a barrier to a peripheral basin ranges from the dominant component of lake water to insignificant, depending on the climate, size of the peripheral basin, and hydraulic conductivity of the barrier. The flux of salt is significant over the full range of modeled hydraulic conductivities, producing brines in the peripheral lake and/or sea within 0.5 m.y. in most cases. This demonstrates that seepage through the barrier can account for lithologic and organic geochemical evidence for saline water conditions, and that marine transgressions across the barrier are not necessary to explain the apparent high salinities.