The History of Sediment Overpressuring in the Gulf of Mexico Basin and its effect on Thermal Maturation

(AAPG Poster Section presentation)

Ulisses T. Mello (1) , Garry D. Karner (2)

(1) Petrobrás Research Center, Cidade Universitária, Qd 7, Ilha do Fundão, Rio de Janeiro, RJ, CEP 21910, Brazil.
Current address: IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598.
(2) Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York, 10964, U.S.A.

INTRODUCTION

One of the most dramatic geological events in the development of the Gulf of Mexico basin has been the rapid, late-stage deposition of Tertiary and Quaternary sediments of the Mississippi delta system. Locally important Mesozoic depocenters also exist and relate primarily to the distribution of extension responsible for the formation of the Gulf of Mexico. Significant geopressures have been created (and dissipated) at various times within the Gulf and in general, track the west to east migration of sediment loads deposited during and after the Cretaceous. The purpose of this poster is to explore the implications of sediment overpressuring in an inelastic compaction-driven fluid flow framework in terms of the temporal behavior of pore pressure and steady-state |and| non steady-state temperatures.

GOVERNING EQUATIONS:

  • 1) Fluid Transfer Equation

    This differential equation describes the physics dealing with the compaction-driven transfer of incompressible pore fluids within a compacting porous medium. We consider a sediment column to consist of two phases: a fluid phase and a solid (or sediment grain) phase. The solid phase is also assumed incompressible. The equation describes the way in which transient fluid pressures tend towards equilibrium via diffusion of pressure. The fluid pressure is generated by both sedimentary loading and thermal expansion of the fluid.

  • 2) Heat Transfer Equation

    This equation describes the movement of heat within a compacting porous media via diffusion and convection. A term for the radiogenic production of heat is also included.

  • 3) Modeling Vitrinite Reflectance

    The thermal maturity of potential source rocks was estimated using a vitrinite reflectance model based on the kinetic equations defining the conversion of organic matter to hydrocarbons. The vitrinite maturation model uses 20 Arrhenius first-order parallel reactions. Our predicted vitrinite reflectance is calibrated in terms of the organic matter "transformation ratio" as defined by Sweeney & Burnham (1990).

  • 4) Porosity and Effective stress

    In soil mechanics, the inelastic behavior of a compaction process is determined from two distinct curves, one dealing with soil compaction and the other with soil swelling. In the case of compacting sediments, we believe that sediments are incapable of significant expansion in response to increasing pore pressure once they have undergone compaction that involved physical damage to the grains and/or pressure solution. Rather, the sediments tend to undergo hydrofracturing. We emulate sediment failure by hydrofracturing when the pore pressure exceeds 0.75 of the lithostatic pressure.

  • 5) Adopted Coordinate System

    Using a fully compacted material coordinate system simplifies the numerical solution of the various differential equations and application to geological problems. The main advantages in this approach are: (1) a constant nodal distance that thereby fixes what would otherwise be a moving and deforming mesh, and (2) each element always embraces the same solid phase of a given sediment. This coordinate system defines the basin configuration when compaction has proceeded to completion (i.e. theoretically when time tends to infinity) and all the state functions such as temperature, pore-pressure, porosity, lithology, etc., are calculated at each nodal position. When the basin geometry or stratigraphy is required for a particular time, these state functions are used to map the time-line stratigraphy of the basin.

  • GEOLOGICAL CONSTRAINTS:

  • 1) Isopach Maps

    The stratigraphic data used in our analysis has been provided by Tectonic Analysis Inc. (unpublished report, Pindell et al, 1990). The regional Gulf stratigraphy was sub-divided into six stratigraphic and hydrologic units. The regional distribution of facies was reconstructed without the complication effects of salt diapirism. As such, the middle Jurassic salt was not included in this analysis. The various isopachs were gridded into 15' x 15' geographic rasters.
    General trends exist in the spatial and temporal patterns of deposition within, and around, the Gulf of Mexico. The distribution of late Jurassic sediments are associated with the rift phase of Gulf of Mexico extension and tend to be concentrated within the interior salt basins (East Texas, Louisiana, and Mississippi salt basins) and across a range of small basins offshore western Florida. Early Cretaceous sedimentation follows approximately the same pattern established by the rift basins but with increased deposition in the West Florida region and the Rio Grande and Campeche areas. Since the late Cretaceous to the Present, a distinctive regional pattern is observed defined by a clockwise migration of deposition across the Gulf of Mexico from southwest to north, and then towards the northeast. Breaching of the barrier reefs along the hinge zone of the Gulf margin allowed the rapid infilling of first the western (Paleocene-Eocene) and then the northern (Oligocene-middle Miocene) and eastern (Pliocene-Pleistocene) parts of the Gulf by fluvial transport and the establishment of deltaic systems of immense thickness (>8 km). Much of the easterly migration in basin depocenter is in response to late Cretaceous Laramide Orogeny and thrust sheet emplacement in the Western Cordillera, high erosion rates and an overabundance of sediment supplied to the Gulf via the antecedent Mississippi-Missouri River system.

  • 2) Distribution of Crustal Extension (beta Map).

    The distribution of crustal extension across the Gulf of Mexico was calculated from the present-day total sediment thickness and bathymetry. To simultaneously match both the observed crustal thickness variations as determined from seismic refraction data with the present-day sediment thicknesses required a pre-extension crustal thickness of 50 km. The onshore regions are characterized by a range in |beta| from 1.2-2.0 whereas the offshore regions show extreme variations. Estimates of 2.0-3.0 occur close to the hinge zone and become greater than 6 in regions that are supposedly floored by oceanic crust. Note that the interior basins are characterized by 1.2 < beta < 1.8. Given the similar tectonic setting of the Newark series of basins with respect to the Gulf interior basins, we assume that |beta| applies only to the crust. In contrast, because of the large degree of extension associated with the offshore basins and the generation of oceanic crust, we assume that |beta| applies to the entire lithosphere seaward of the hinge zone.

  • 3) Localization map

    Three north-south regional profiles were selected to define the spatial and temporal development of the Gulf of Mexico at 264*, 268* and 272* longitude. In these profiles, the depth-coordinate is with respect to sea bottom, and thus to obtain the actual depth to an interface, the respective bathymetry or paleobathymetry must be added. In addition, we have created a theoretical "well" at 28* lat. and 268* long. by displaying geohistory and maturation information in a more familiar format.

  • PREDICTING OVERPRESSURE: CRITICAL PARAMETERS

    We modeled the generation and destruction of abnormal sediment pore pressures due to variations in sedimentation rate, facies type, and formation porosity and permeability using a finite-element analysis to solve the coupled differential equations of both heat and fluid transport in a "fully compacted" sediment matrix system. Critical parameters in this analysis are the sediment specific storage, hydraulic conductivity and hydraulic diffusivity. Many of these parameters have equivalents in the equations describing the conductive-advective flow of heat. We will discuss each of these "sediment" parameters in turn:

  • Specific Storage:

    We assume initially that the porosity-effective stress relationship within the sediment follows a negative exponential steady-state form. Specific storage is the property of sediment to retain fluids during pore pressure variations. It is formally defined as the amount of fluid expelled due to a unit increase of hydraulic head. The specific storage for a sediment in which the interstitial fluids are incompressible tends to decrease with depth due to the decrease of porosity with depth (or equivalently, the decrease in bulk compressibility with depth).

  • Hydraulic conductivity:

    Hydraulic conductivity defines the rate at which a fluid can pass through a porous medium. The hydraulic conductivity is directly proportional to the sediment permeability which can feasibly decrease 3 orders of magnitude within the upper sections of a sedimentary basin.

  • Hydraulic diffusivity:

    Hydraulic diffusivity, analogous to thermal diffusivity, essentially describes the rate of transfer of pressure across a unit area of sediment. Hydraulic diffusivity is defined as the ratio of hydraulic conductivity and specific storage. As such, hydraulic diffusivity has an interesting behavior with depth, defining the shallowest possible position of the top of the overpressure (ranging from 2 to 4 km).

  • APPLICATION TO THE GULF OF MEXICO

    The generation of overpressure is caused mainly by the rapid deposition of sediment. The inability for pore pressure to escape at a rate commensurate with sedimentation leads to sediment overpressuring. In the ensuing figures, we compare and contrast the development of sediment overpressures and porosities induced by rapid, late-stage deposition of Quaternary sediments of the Mississippi delta system along a series of north-south cross-sections of the Gulf of Mexico (at 264*, 268* and 272* long.) and a theoretical "well" at 28* lat. and 268* long.

    TIME-DEPTH SECTIONS: A THEORETICAL WELL

    The selected well represents a typical geological situation in the Gulf Coast region where massive progradational sands overlie sands and intercalated shales which in turn overlie massive shales. The accompanying figures are time-depth diagrams generated by forward modeling the 1-D development of the Gulf (as opposed to time-depth plots obtained by backstripping). As can be seen from the isopach maps, the highest sediment deposition rates in this region occur during the Miocene and Quaternary. Our figures summarize the effect of varying sedimentation on the hydraulic head, porosity, temperature and vitrinite reflectance as functions of time.

    Overpressures >0.75 kpsi (i.e., a hydraulic head of 500 m) began to develop in the Campanian (83 Ma) at depths of approximately 2.5 km. However, significant pore pressures were developed in the Miocene (22 Ma) with a maximum hydraulic head of 9.4 km (-> 13.6 kpsi) predicted for the present day. The thermal effect of this large overpressuring associated with sediment undercompaction increased the temperature gradients from the Miocene to the Present despite the fact that the basal heat flow engendered by extension continued to decrease through time.

    A rapid increase in temperature gradient can also be induced by processes other than overpressuring. For example, the relatively rapid increase of temperature gradient at shallow depths during the early Cretaceous is a consequence of the different compaction characteristics of shales, sandstones and carbonates. In this case, the temperature gradient variations are associated with the change in facies from sands to shales across the early Cretaceous shelf (i.e., these shales represent the "classic" thermal blanket). The predicted vitrinite reflectance follows closely the behavior of the isotherms with the onset of the oil window (0.6 %Ro) being close to 110+-10 *C isotherm whereas the peak of oil generation (1.0 %Ro), the end of oil generation (1.35 %Ro) and the limit of wet gas preservation (2.2 %Ro) all following approximately the 150+-10 *C, 170+-10 *C, and 210+-10 *C isotherms, respectively.

    HYDRAULIC HEAD

    In the Gulf of Mexico, the generation of overpressure is intimately linked to the sedimentation rate. For example, at the end of the Neogene, rapid sedimentation related to sediment input from the Mississippi delta system generated a high hydraulic head (~ 9 km -> 13.4 kpsi). As the sediments continued to prograde south into the Gulf Coast area, maximum overpressures shifted towards the Sigsbee Plain area (values of ~13.5 km| -> 20.3 kpsi) with the top of overpressure being approximately at a depth of 3 km.

    On the other hand, the dissipation of overpressure depends on the hydrological properties of the sediment (i.e., porosity and permeability). The relationship between the development of overpressure and sediment load can be readily appreciated in the modeled sections, especially close to the Paleogene shelf break. Since subsequent Neogene and Quaternary sediments have bypassed this part of the margin, no significative load has been emplaced on the margin and therefore no overpressuring is generated.

    Overpressures are predicted in upper Jurassic and lower Cretaceous rocks in the western part of the Gulf basin beginning in the Paleogene (see prediction of hydraulic head figure for the Paleogene; 264* long). In general, overpressuring is an important characteristic of the Rio Grande and West Texas areas since the Maastrichtian and is not restricted to Quaternary sedimentation in the Gulf Coast basin. The maximum values of overpressure are always developed close to the basement with the top of overpressure rarely being shallower than |2 km.

    POROSITY

    Undercompaction is associated mostly with Quaternary, Neogene and Paleogene overpressured sediments. Although maximum overpressuring is predicted to occur in the deepest part of the basin, the sediments are in fact normally compacted (to the point of even approaching completion). For example, the overpressuring predicted within the normally compacted late Jurassic sediments is due to: (1) the sealing effect of the overlying early Cretaceous shales and carbonates compounded by (2) the amplification of pore pressures by the Miocene-Quaternary sediment load. This is because compaction is assumed to be geologically unrecoverable (i.e., it is an inelastic process). In contrast, both overpressuring and undercompaction are predicted within the Quaternary-Miocene section of the Gulf Coast basin.

    TEMPERATURE

    Our modeling predicts that by the end of the Neogene, temperatures have almost reached steady-state (i.e. equilibrium) over broad regions of the Gulf Coast basin. Consequently, the highest temperatures occur in the deepest parts of the basin. However, during the Quaternary, rapid progradation of cold, high porosity sediments loaded the pre-existing sections and induced additional subsidence in the region of the Sigsbee Plain. The high porosity of this sediment creates an anomalously low thermal conductivity and thus acts as a thermal insulator to the conductive flow of heat. This Quaternary section has yet to reach thermal equilibrium and so will be anomalously cold with respect to its depth. That is, even though the Neogene section may be deeper than its counterparts in the northern Gulf Coast basin, it will tend to be colder.

    VITRINITE REFLECTANCE

    The calculated vitrinite reflectance indicates that for most of the basin history, the top of the oil window remained at approximately 3 km. Similarly, the base of the oil window ranged from 4 to 6.5 km. At the present day, the depth to the top of the oil window is strongly affected by the rapidly deposited Quaternary sediments.

    FACTORS CONTROLLING THE TOP OF GEOPRESSURE

    A physical explanation for the Top of Geopressure (overpressure) in the Gulf Coast area is the based on the behavior of hydraulic diffusivity with depth that shows a minimum at depths ranging from 2 to 4km due to high porosity and permeability a shallow depths and very low bulk compressibility (or equivalently specific storage) at deeper sediments.

    CONCLUSIONS

  • Overpressuring can be produced in both normally compacting and undercompacted sediments. For example, sediment that has been normally compacted and is approaching completion (i.e. < 5%), and then loaded from above, will develop overpressuring. In contrast, rapidly deposited sediments will be both overpressured and undercompacted. Only the overpressure associated with undercompaction can result in thermal anomalies.

  • Thermal anomalies within undercompacted sediments, when referred to a steady-state temperature profile for normally compacting sediments, are initially negative and become positive as the temperature structure approaches equilibrium.

  • Elastic compaction models (i.e. the usual model applied to compacting sediment systems) |always| overestimate the temperature and therefore levels of maturation. That is, the compaction process is not reversible.

  • The Top of Geopressure in regions of rapidly depositing sediments is explainable in terms of a "barrier" or minimum in hydraulic diffusivity at depths of 2.5 to 4 km depth.

  • Contrary to conventional wisdom, the generation and maintenance of overpressuring is not a feature restricted to the Quaternary and Miocene sediments of the Gulf Coast basin. In fact, significant overpressures exist today within the Rio Grande and West Texas area and were initiated in the Maastrichtian.