Basin Research - Volume 7, Issue 1, 1994

Chemical and isotopic data on formation waters of oil‐fields from two parallel anticline axes of the Mahakam Delta Basin provide information about the present‐day mass transfers in the sedimentary sequence of this basin. Depletions in Ca, Sr and K, enrichment in Rb, and the Sr/Ca ratios in the waters are related to illitization of smectite and precipitation of carbonate minerals, as well as dissolution of K‐feldspar and precipitation of albite. These short‐lasting processes seem to have been more pronounced in the deeper segments of the proximal anticline. The Sr, oxygen and hydrogen isotopic data emphasize occurrence of lateral migrations of the formation waters within permeable units from Borneo Island to the proximal anticline under continental influence, and further to the distal anticline under marine influence.

Description of a combined process including illitization of smectite, precipitation of carbonate minerals, dissolution of K‐feldspar and precipitation of albitic feldspar, during lateral migrations of the formation waters was only possible because of the young age and the restricted volume of the basin. In addition, the chemical signatures in the formation waters were not obscured by water‐rock interactions during long‐distance migrations that occur systematically in large sedimentary basins.

One of several interconnected depocentres lying offshore eastern Canada, the Sable sub‐basin preserves a thick sequence of Mesozoic‐Cenozoic clastic sediments, significant gas accumulations and an extensive development of abnormal pressures. In order to understand the basin's hydrocarbon generation, migration and accumulation history it is necessary to quantify the interplay between its burial, thermal, and maturation history, and to determine the influence on these of the basin's excess pressure history. Simple, one‐dimensional reconstructions of maturity and pore pressure histories are derived for exploration well and pseudo‐well locations on a seismic line running from the basin's structural high to its depocentre. Calibrated, where possible, by reference to measured maturity, temperature and pressures, these models provide a basic dynamic framework within which it is possible to consider the generation history of the basin's source rocks. Late Jurassic to Early Cretaceous sediments underwent an initial rapid, rift‐related subsidence. The thermal/maturation models suggest that source rocks lying within these intervals quickly matured and began generating gas and condensates. Similarly, this rapid burial was translated, through sediment compaction disequilibrium processes, into an early expression of abnormal pressures.

The pore pressure/time reconstructions in the modelling assume that sediment compaction disequilibrium and gas generation are the principal causal mechanisms. Matching pore pressure reconstructions with present‐day pressure‐depth profiles is particularly sensitive to assumed seal permeability profiles. Although the seal permeabilities used as model input are based on actual measured permeabilities at the present day, this does not mean that the permeability‐time curves derived through the model's decompaction assumptions accurately reflect seal permeability evolution.

Unconformities, which represent either periods of interruption of sedimentation or, in most cases events characterized by deposition and subsequent erosion, are commonplace geological phenomena in sedimentary basins, and will affect the pore pressure evolution of the basin fill. The effect of unconformities on pore pressure, as well as on sediment compaction and on burial processes is studied using a numerical basin model. For coarse sediments, which are permeable so that their pore pressure always remains nearly hydrostatic, the effects of both pure deposition interruption (hiatus) and deposition‐erosion events are negligible for pore pressure evolution. However, for fine‐grained sediments, unconformities can modify the pore pressure and the stress state to varying degrees. The results show that the rate of removal of overlying sediments, the permeability of sediments and time play important roles in the pore pressure evolution. In the East Slope of the Ordos Basin (China), in which overpressure has not been detected in deep wells, the modelling results suggest that the large‐scale erosion occurring in the Late Cretaceous and in the Tertiary may have removed high overpressure existing in the basin before the erosion.

Cross formational flow of water has occurred across several hundred metres of Liassic mudstone from the Triassic Chaunoy Formation sandstone to the Middle Jurassic Dogger Formation carbonate in the Paris Basin, France. This has been demonstrated by chemical and isotopic data from rocks and formation waters, sampled on a basin scale, from the Dogger and Chaunoy formations. Present‐day and palaeoformation waters in the Dogger record input of exotic water in terms of salinity and carbon and strontium isotopes. The exotic water was highly saline and contained isotopically light carbon and ⁸⁷Sr‐enriched strontium in comparison to the indigenous Dogger Formation water. The source of the exotic water can only have been the Chaunoy Formation. Salinity and isotope data show that present‐day Dogger Formation water contains 10–15% invasion Chaunoy water. Modelling strontium isotope ratio and concentration data from mineral cements shows that over the duration of Dogger cementation, Dogger palaeoformation waters were replaced by 1–5% Chaunoy water. Cross formational flow of water must have occurred by convective mass flow, rather than diffusion, in order to preserve the characteristics of water input from the Chaunoy. Flow probably occurred via the major Hercynian fault and fracture systems that run through the centre of the Paris Basin. Upward flow in this setting is most likely in a compressional tectonic regime, the situation during much of the Tertiary in the Paris Basin. Thermodynamic modelling indicates that cross formational flow from the Chaunoy has probably caused carbonate cementation in the Dogger.

In the Earth's crust the temperature is largely controlled by heat conduction. However, under some circumstances, the thermal state is disturbed by advection of heat associated with groundwater flow. The corresponding thermal disturbance depends on the water flow velocity (modulus and direction) and therefore thermal data may be used to constrain the pattern of natural fluid flow. In this paper, some models of thermal disturbance induced by convective heat transfer are presented. They are based on the assumption that the water flow is concentrated in thin permeable structures such as aquifer or fault zones. The steady‐state and transient thermal effects associated with such scenarios are computed using a somewhat idealized model which depends on a small number of parameters: flow rate, time, aquifer geometry and thermal parameters of surrounding rocks. In order to extract the conductive and convective components of heat transfer from temperature data and to estimate the corresponding fluid flow rate, it is first necessary to estimate the thermal conductivity field. The problem of the estimation of thermal conductivity in clay‐rich rocks, based on laboratory and in‐situ measurements, is emphasized. Then a method is proposed for the inversion of temperature data in terms of fluid flow. Vertical and lateral variations of thermal conductivity are taken into account and the fluid flow is assumed to be concentrated on a specified surface (2‐D quasi‐horizontal pattern). Thermal effects of the flow are simulated by a distribution of surface heat production which can be calculated and then inverted in terms of horizontal fluid flow pattern.

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 suggests that ancient groundwater flow systems can be detected by comparing thermochronometric data between suspected recharge and discharge areas within continental rifts. Vitrinite reflectance profiles, observed offsets in the depth to the onset of petroleum generation, and apatite fission track annealing studies are all well suited for detecting groundwater flow systems which have been relatively long lived (10⁷ years). Apatite fission track age data are probably best suited for identifying ancient groundwater flow systems within rifts long (>200 Myr) after flow ceases.

Mineral provinces in southern and central Africa are strongly controlled by major structural trends, the alignment of sedimentary basins and metamorphically induced thermal regimes deep in the crust. Ore deposits are preferentially located on cooler margins of orogenic belts and are ultimately end‐products of fluid expulsion out of the deeper parts of orogenic axes. Metamorphic and structural vectors within orogenic belts adjacent to major cratons show a trend of high‐grade thermal overprinting in the cores of axes and lower metamorphic regimes acting on the distal margins of orogens (e.g. foreland basins). This apparent pattern is considered to have importance in the expulsion of at least three generations of mineralizing fluids beginning with exhalative migration during diagenesis and culminating much later in thrust‐controlled expulsion onto adjacent craton margins. Fluids within the hydrosphere that accumulate initially through topographic gradients in the sediments mixed with components of the mantle (CO2). After storage within the crust, migration, enforced by metamorphic processes, transferred fluids out of, and away from, high thermal regimes in the axes of belts, leading to their present preservation around the margins of the major cratonic nuclei.