A survey of rift basin source rocks

Abstract

Various studies reveal that rift basins contain a disproportionate amount of petroleum based on their area and sediment volume. In part, this results from the excellent petroleum source rocks found in many of these basins. The presence of these sources not only influences the petroleum potential of the rift sequence, but may also influence the petroleum potential the post-rift succession. These high quality source rocks can be partially explained by the low width:depth ratios and high subsidence rates associated with rift basins. However, not all rifts contain these organic-rich deposits, nor are all the organic-rich deposits volumetrically significant. Other factors, such as climate and sedimentation rate, strongly influence the deposition of these organic-rich sediments. Within individual basins sub-basins may display dramatically different oil-vs gas-prone tendencies due to different subsidence rates. High subsidence rates favour oil source rock development, while low subsidence rates favour gas source development. Many, but not all, of these petroleum source rock systems are lacustrine. Restricted marine source rocks are also present within rift sequences. This study focuses on three elements of rift basin source rocks: (1) their distribution; (2) the factors controlling their deposition, distribution and quality; and (3) their geochemical attributes.

Petroleum systems of Indonesia

“…These consist of an association of alluvial, shallow to deep lacustrine and fluvio-deltaic facies represented by laminated shales, silts and sands with coals and conglomeratic intervals. Deep lake organic rich shales containing algal/amorphous material with thin sands (Brown Shale Formation), and shallow lake light grey shales with humic coals ensure that charge from the early synrift is mixed lacustrine and terrestrial, mainly type I/II, within which four oil families have been distinguished (Katz, 1995). The best reservoirs are found in fluvio-deltaic sands, where porosities and permeabilities may be up to 17% and 100 mD, respectively.…”

A surprising asymmetric paleothermal anomaly around El Gordo diapir, La Popa Basin, Mexico

“…Locally present and adjacent to the evaporite diapirs are shallow-marine carbonate lentils, which are defined in the North American Stratigraphic Code as geographically restricted members that terminate on all sides within a formation, composed of oyster-rich calciclastic deposits and coral-and red algal-dominated reefs that rimmed the diapirs during near-surface diapiric rise (Hunnicutt, 1997(Hunnicutt, , 1998Mercer and Giles, 2000;Giles, 2003, 2004). McBride et al (1974) and Wolleben (1977) thought that the basin strata were almost entirely Cretaceous in age, but new biostratigraphic studies (Vega and Perrilliat, 1989a, b, 1992, 1995Vega-Vera et al, 1989) indicate that the upper part of the Difunta Group in La Popa Basin ( Figure 3) is Paleogene. The Delgado Sandstone, which separates the Upper Mudstone and Lower Mudstone Members of the Potrerillos Formation, is recognized as being linked to the Chicxulub impact that marks the Cretaceous-Paleogene (KP) boundary (Lawton et al, 2005).…”

“…Either lacustrine or marine organic-rich anoxic sediments, and sometimes both lacustrine and marine sediments, are frequently deposited in rift basins and provide excellent source rocks (Katz, 1995). Rift basins commonly have high heat flow that contributes to source rock maturation (Morgan, 1982;Lysak, 1992).…”

Hydrocarbon potential of the Meso-Cenozoic Turkana Depression, northern Kenya. II. Source rocks: quality, maturation, depositional environments and structural control

“…Selected pieces of shale were as rich as 11% . Following a standard continental rift, half-graben sedimentation model (Katz, 1995;Lambiase & Bosworth, 1995), it was anticipated that the source rocks would thicken considerably towards the basin centre, and that their source quality would be maintained or improve in the same direction.…”