Journal of Petroleum Geology最新文献

Fibrous calcite bed-parallel veins (BPVs) are a typical feature of the Upper Jurassic – Lower Cretaceous Vaca Muerta Formation in the subsurface of the Neuquén Basin (Argentina). The formation is considered to be the main source rock in the basin as well as an important unconventional play. This study examines the growth of BPVs through an analysis of core from three wells located along a transect extending for some 150 km from the NE Platform near the basin margin in the east to the Agrio fold-and-thrust belt at the Andean deformation front in the west. The main objective is to integrate fluid inclusion data with the palaeothermal and palaeopresure evolution obtained from a regional-scale 2D basin and petroleum systems model to examine the timing of fracture development and its relationship with hydrocarbon generation in the Vaca Muerta Formation through time.

The apertures of BPVs were measured in more than 360 m of core from three wells (wells A, D and E). This data was combined with optical petrography to investigate the number of calcite cementation events, and the temperature of cement precipitation based on fluid inclusion data. The organic geochemical and mineralogical characteristics of the Vaca Muerta source rock were also analysed. The integrated results were incorporated into a poro-elastic basin model to investigate the impact of horizontal shortening due to Andean compression on pore pressure development and fracturing in the Vaca Muerta Formation. This framework allowed the timing of BPV formation to be determined together with possible mechanisms governing overpressure conditions through time.

Near the Andean deformation front in the west of the modelled section where the Vaca Muerta Formation is in the wet gas window (well D) and dry gas window (well A), BPVs are characterized by two or more generations of calcite fibres indicating multiple growth phases. Calcite which precipitated during cementation event 1 (E1) in the internal zones of BPVs consists of crystals oriented perpendicular to fracture walls, indicating perpendicular vein opening. Calcite precipitated during cementation event 2 (E2) in the outer zones of BPVs includes curved and oblique crystals. During this phase, shear occurred between the opening vein walls as a result of horizontal shortening. Cementation event 3 (E3) is characterized by an equant mosaic of calcite crystals which preserve intracrystalline porosity. E1cements formed between 110 and 90 Ma with trapping temperatures of ∼112 °C (upper Vaca Muerta, well A) and ∼125 °C (lower Vaca Muerta, well D). Fracturing resulted from disequilibrium compaction and from volumetric expansion due to primary cracking of kerogen within the oil window. E2 cements record a trapping temperature of ∼159 °C and formed between 70 and 55 Ma (lower Vaca Muerta, well D) during maximum burial of the Vaca Muerta Formation, synchronous with the secondary cracking of retained liquid hydrocarbons and the beginning of An

The synrift Mayo Oulo-Léré Basin in Northern Cameroon is located in the transition zone between the West and Central African Rift Systems. Structural and stratigraphic elements of the basin resemble those of the Yola Basin in NE Nigeria, an extension of the Upper Benue Trough. The Lower Cretaceous lacustrine shales with source rock potential which occur in nearby rift basins are also present in the Mayo Oulo-Léré Basin. These shales were investigated at two outcrop locations (Badesi and Tchontchi), and samples collected (n = 60) were subjected to palynofacies and bulk geochemical analyses to evaluate their petroleum generation potential and to interpret their depositional environment. At the studied locations, shales were divided into two lithofacies: grey to black laminated shales containing algal-bacterial OM together with common woody (phytoclast) debris (“facies Fml”); and cm-bedded shales which had a higher content of algal-bacterial OM but a lower phytoclast content (“facies Fmlc”).

Palynological and bulk geochemical data indicate that the shales contain well-preserved organic matter (OM) and locally display good to excellent oil generation potential. Average TOC contents are 2.7% and 1.4% for samples of the Fmlc and Fml facies shales respectively. HI values (94-889 mg HC/g TOC and 131-638 mg HC/g TOC respectively) suggest that the shales contain Types I to III kerogen. Organic material in the Fmlc facies shales is dominated by amorphous organic matter (AOM: 90% on average) with a low phytoclast content (6% on average); whereas samples of the Fml facies shales contain less AOM (74% on average) and have a higher phytoclast content (23% on average). AOM in the Fmlc shales is highly fluorescent; these shales are interpreted to have been deposited in dysoxic to anoxic conditions. The AOM in the Fml shales is weakly fluorescent and the shales were deposited under more oxic conditions. The kerogen in the shales ranges from immature to early oil window mature. Average values of the pyrolysis S2 yield are 15.5 mg HC/g of rock and 7 mg HC/g of rock for samples from Fmlc and Fml facies shales respectively. The shales increase in thickness northwards towards the Logone Birni Basin where they may have reached the oil window, as in neighbouring basins. The results of this study of lacustrine shales from the Mayo Oulo-Léré Basin suggests that there may be potential for oil exploration in northern Cameroon.