Burial history/rate

In all the above equations the rate of a reaction is seen to be temperature dependent, and the equations only apply to isothermal conditions. In geological systems, complex burial histories are usually involved, with variable rates of heating and possibly also intervals of cooling. For such systems the rate equations require integration with respect to both temperature and time, and it is assumed that Avalues are little affected by temperature change (Lewis 1993). 

Figure I. Elements of a Petroleum System. All petroleum systems contain 1. at least one formation of organic-rich sediments that has been buried to a sufficient depth by overburden rock such that petroleum is generated and expelled, 2. Pathways (permeable strata and faults) that allow the petroleum to migrate, 3. Reservoir rocks with sufficient porosity and permeability to accumulate economically significant quantities of petroleum, and 4. Sealing rock (low permeability) and structures that retain migrated petroleum within the reservoir rock. The top and bottom of the oil window is approximated as a function of burial depth. In actual basins, these depths are not uniform and vary as a function of organic matter type, regional heat flow from basement, in thermal conductivity of the different lithologies, and burial history (e.g., deposition rates, uplift, erosion, and hiatus events).

The reconstruction of the burial and thermal history of the Namorado Sandstone was performed using the BaSS software (Basin Simulation System) developed by Chang et a/. (1991). The calibration of the thermal history was carried out by measured vitrinite data and smectite/illite conversion rate. The amount of illite in the interlayered I/S clays was determined via XRD analysis. The organic matter residue of the associated shales close to the reservoir intervals was petrographically analysed to obtain the vitrinite index. Depth in the diagrams and tables is referred to datum level (driller-measured depth). 

Even though this sediment represents marine offshore deep-water turbidites, meteoric flow could have reached the reservoir just after turbidite deposition, during a sea-Ievel lowstand (Fig. 13), when both the platform and part of the slope were partially exposed. The relatively low burial rate during the early history of the Namorado Sandstone favours this hypothesis. Meteoric waters resulted in the formation of blocky calcite that replaces bioclasts or occurs as the inner rims of mouldic pores (see Carvalho et al., 1995). 

The model simulates the history of shaly source rock for an assumed burial and thermal history. The results discussed here are based on a constant sedimentation rate of 500 m/m.y., and a geothermal gradient of either 30 C/km or60 C/km, A Gulf Coast-type porosity gradient for shale is used, where porosity = 0.39 depth (ft) increment... 

Complex exposure histories may also show up as a disturbed pattern of cosmogenic nuclide concentration versus depth. Particular care must be taken when dating soil or alluvial deposits, which often experience varying sedimentation rates, sudden burial, or bioturbation (i.e., soil mixing by living organisms) of the uppermost layers (e.g., Phillips et al. 1998 Braucher et al. 2000). In such studies it is extremely important that depth profiles are taken. 

The goal of these studies is to link the paragenesis of these (and other) phases to events in a rock s history which, when combined with texture-sensitive geochronology (e.g., SIMS, LA-ICP-MS, and EMP), will enable the determination of the time of these events. These data will allow direct inferences of fundamental tectonic parameters such as the duration of metamorphic events, the duration of melting in the crust, the rates of heating/cooling and burial/exhumation of orogenic belts, and the time and duration of hydrothermal activity. 

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