One‐dimensional models of groundwater flow, sediment thermal history and petroleum generation within continental rift basins

Abstract

We investigate the effects of convective heat transfer on the thermal history of sediments and petroleum formation within continental rift basins using one‐dimensional mathematical modelling. The transport equations used in this study to describe vertical groundwater flow and conductive/convective heat transfer are solved by the finite element method. Sediment thermal history is quantitatively represented using first‐order rate kinetic expressions for kerogen degradation and an empirical fanning Arrhenius model for apatite fission track annealing. Petroleum generation is also represented in the model by a suite of first‐order rate kinetic expressions. The analysis provides insights into how pore fluid circulation patterns are preserved in the rock record as anomalies in palaeogeothermometric data within continental rifts. Parameters varied in the numerical experiments include the ratio of conductive to convective heat transfer (thermal Peclet number; Pe) and the composition of the disseminated organic matter in the sediment (type II and III kerogen). Quantitative results indicate that vertical groundwater flow rates on the order of a mm/yr cause a change in computed vitrinite reflectance of the rocks and a shift in the depth to oil generation by as much as 3000 m. Differences in thermal gradients between recharge and discharge areas (Pe= 0.6) also change the width of the zone of oil generation by a factor of two. Even more dramatic, however, are the large changes in predicted apatite fission track length distributions and model ages between recharge and discharge areas. For example, a sediment package buried to a depth of 2400 m over 200 Myr within the groundwater recharge column had a fission track length distribution with a computed mean and standard deviation of 12.83 μm and 0.77 μm, respectively. The fission track model age for this sediment package was 209 Ma. The same sediment package in the discharge area has a distribution with a mean track length of 5.68 μm, a standard deviation of 3.37 μm, and a fission track model age of 2.6 Ma. Transient groundwater flow simulations, in which fluid circulation ceases after a period of time within the rift basin, are also presented to illustrate how disturbances in palaeogeothermometric parameters are preserved on geological time‐scales. Vitrinite reflectance profiles require about 10 Myr to return to conductive conditions within groundwater recharge areas while the convective disturbances are preserved indefinitely along the discharge column, as long as further subsidence does not occur. Ancient groundwater flow systems are preserved as anomalies in computed apatite fission track model ages and distributions much longer after groundwater flow stops, relative to organic‐based geothermometers. Significant differences exist in model ages between recharge (145 Ma) and discharge (90 Ma) areas 200 Myr after flow has ceased. However, calculated fission track histogram distributions are virtually identical in recharge and discharge areas after about 50 Myr. Our study...

Preferential petroleum migration pathways in eastern Pearl River Mouth basin, offshore South China Sea

“…Moreover, the overall structural configurations have remained unchanged, which would exclude either major influences of topographydriven groundwater flow on petroleum migration (Bekele et al, 2002;Toupin et al, 1997;Person et al, 1996Person et al, , 1995Person and Garven, 1992;Garven, 1989) or postaccumulation alteration of oil reservoirs (e.g., Hao et al, 2008;Zou et al, 2008). (4) Most reservoirs are close to hydrostatic pressure, which exclude possible influences of overpressure build-up on fluid flow and petroleum migration (e.g., Hao et al, 2007b;Hao, 2005;Holm, 1998).…”

Simulation-enhanced fracture detection: research and demonstration in U.S. basins

“…A fundamental component of most existing basin models is petroleum generation Forbes et al 1992;Person & Garven 1992;Bredehoeft et al 1994;Person et al 1995;Burrus et al 1996;Mello & Karner 1996;Forster et al 1998;Schegg et al 1999;Lee & Williams 2000;Wendebourg 2000;Payne & Ortoleva 2002a,b). The generated petroleum is used as a lumped source term in the mass balance equation for the oil or gas phase, i.e.…”

“…(1) Conventional basin models that have the capability to simulate multiphase flow, but use empirical laws for compaction Forbes et al 1992;Person & Garven 1992;Person et al 1995;Wieck et al 1995;. Some of these models include petroleum generation .…”

“…Classical advective±dispersive heat transfer equation is used to model heat ow Forbes et al 1992;Person & Garven 1992;Person et al 1995;Wieck et al 1995;Mello & Karner 1996;Forster et al 1998;Polyansky & Poort 2000;. A speci®ed temperature is imposed at the interface between sediment pile and sea, and heat¯ux history is applied at the bottom of the sediment pile.…”

“…An important limiting assumption made in most basin models is that there exists a simple dependence of porosity on effective stress Forbes et al 1992;Person & Garven 1992;Wieck et al 1995;Mello & Karner 1996;Schegg et al 1999;Lee & Williams 2000;Polyansky & Poort 2000; or depth (Bredehoeft et al 1994;Person et al 1995;Burrus et al 1996). Different compaction parameters are used for shales and sandstones regardless of their mineral composition, and grain size distribution.…”