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Tropical
versus Temperate Zone Lacustrine Source Rocks:
Examples from Takutu Basin, Guyana, and General Levalle Basin, Argentina
by
Robert E. Webster1
Search and Discovery Article #10070 (2004)
*Adapted from a poster presentation at AAPG Annual Convention, Dallas, Texas, April 18-21, 2004.
1Hunt Oil Company, Dallas, Texas ([email protected])
Rift basin lacustrine source rocks have generated 5.7 percent of the world’s reserves, and some of today’s most active exploration plays are driven by lacustrine-shale-based petroleum systems. However, paleolatitude and paleoclimate play major roles in determining organic richness of lake sediments, and not all restricted lacustrine basins contain commercial petroleum systems. Two continental-interior lacustrine rift basins in South America display dramatic differences.
In the Takutu basin, at a tropical paleolatitude of
2°N, deposition in an Early to Middle Jurassic rift during global “hothouse”
conditions produced organic-rich shale with 1.5 to over 4% TOC, hydrogen indices
of over 360, and pyrolysis yields up to 9300 ppm. Terrestrial kerogen is
significant, but Type I/II non-marine algal amorphous kerogen is dominant in
richer intervals. Significant oil and gas were generated but mostly lost during
Miocene(?) tectonic reactivation.
In
contrast, the Lower Cretaceous General Levalle rift basin formed at a temperate
zone paleolatitude of 45°S during mild “icehouse” climatic conditions. Dark
lacustrine shale is organic-lean, with maximum TOC just over 1%. Kerogen is Type
III/IV, derived mostly from pollen from nearby upland forests, with little
generative capacity. Here the annual seasonal cycle probably caused lake
turnover and prevented long-term establishment of an anoxic bottom layer to
preserve organic matter, which is common in more tropical settings such as the
Takutu.
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Introduction
Purpose of Article/Poster Presentation
Global Considerations: Lacustrine Source Rocks
The depositional model for a restricted lacustrine basin is shown in Figure 1.1. This representation is for the General Levalle basin of Argentina.
Summary of Features of Two South American BasinsThe locations of the General Levalle Basin in Argentina and the Takutu Basin in Guyana/Brazil are given in Figure 1.2. Major features of each are summarized below.
Features of General Levalle Basin, Argentina (Figure 1.3)
Features of Takutu Basin, Guyana and Brazil (Figure 1.4)
General Levalle Basin, ArgentinaFigure Captions (2.1-2.12)
Geologic Setting (Figures 2.1-2.5) The General Levalle Basin forms a long, narrow, and deep Lower Cretaceous intracratonic rift in southern Cordoba province, Argentina. As a buried Lower Cretaceous rift, it trends approximately north-south for over 150 km, ranges from 5 to 50 km in width, and is over 6500 m deep (Figure 2.1). Below a prominent mid-Cretaceous unconformity, steeply dipping normal faults bound tilted graben and half-graben fault blocks (Figure 2.3). The lower rift-fill section, the General Levalle Formation (Figure 2.2), is a Valanginian-Hauterivian-aged clastic-evaporite package, over 3200 m thick. It was deposited in an arid, restricted, rift basin that included a hydrologically-closed saline lake. Nine lithology-based members represent one continuous cycle of deposition, with a lower coarse clastic sequence gradually fining upward into an evaporite member and then coarsening upward again into an upper sandstone. The uppermost rift fill sequence, the Guardia Vieja Formation, is a series of Aptian basalt flows and sills over 800 m thick with some clastic interbeds. Unstructured Pleistocene to Upper Cretaceous strata overlie the buried rift basin. In 1995-96, the first exploratory well in the basin (Figures 2.4 and 2.5) tested a deep-seated anticline to 5179 m but encountered just one minor show. Reservoir-quality sandstone occurs only in the upper rift sandstone member, but this lacked adequate seals. Basin-center dark shale below the evaporite member was thin, surprisingly low in TOC, and overmature. Given the narrow, deep depocenter, unfavorable reservoir-seal relationships, and the lack of source facies, an effective petroleum system remains unproven in the basin.
Paleogeography and Climate: Different Perspectives Late Cretaceous paleogeography, portrayed by Christopher Scotese (Figure 2.6) shows the extent of the Tethys Ocean and the expanding Atlantic Ocean. Interpretations of Early Cretaceous paleogeography are shown in Figure 2.7, from Ron Blakey, and in Figure 2.8, from Robertson Research. They both show General Levalle Basin to be intracontinental. Mild “ice house” conditions existed in southern Gondwana in the Early Cretaceous, probably with snow and ice during the winter season, as shown in Figure 2.9, from Christopher Scotese.
Source Rock GeochemistryAnalysis of data and tests from Hunt Cd. GL x-1 well indicate that the petroleum system in General Levalle Basin is poor. A summary of the results is given in Figures 2.10 and 2.11, and the drilling results are illustrated in Figure 2.12. The drilling results of Hunt Cd. GL x-1 are listed below: · Lower Cretacous (Neocomian) rift fill. · Poor source rock found: Lean Type III/IV. · Lower rift shales overmature for oil. · Only 1 minor oil show. · Good evaporite seal in middle rift section. · Reservoirs below seal tightly cemented.
Takutu Basin, Guyana and BrazilFigure Captions (3.1-3.13)
General SettingThe Takutu basin is an ENE-trending Jurassic-Early Cretaceous continental (lacustrine) rift basin, about 40 km wide and 280 km long, that cuts the Guyana shield in southwest Guyana and northern Brazil (Figure 3.1). Prior exploration documented a stratigraphic section dominated by mudstone but including Jurassic lacustrine source shale, siltstone, evaporites, and basalt (Figure 3.2). Numerous anticlinal and tilted fault block structures, including a noncommercial oil discovery (Figure 3.3), suggested an attractive exploration play existed. In late 1988 Hunt Oil Co. began operations in the basin. A three-year exploration program included field geology, photogeologic mapping, several methods of surface geochemical prospecting, reprocessing, and acquisition of SAR, aeromagnetics, and 1331 km of new seismic. Exploration efforts ultimately focused on the large basin-center Savannah Arch(Figures 3.3, 3.4, and 3.5). An anticline near the south end, where the exploration well, Turantsink 1 (Figure 3.5), was drilled, was interpreted as a drape feature above a thick lacustrine-fan-delta complex. However, the deep structure proved rooted in a thickened salt section near the Jurassic basin paleocenter. Minor oil shows were observed at several horizons, but the predicted sandstone reservoirs are not present. This part of the basin had been affected by a Tertiary hydrothermal event that drove the thick source shales into overmaturity and destroyed porosity in all potential reservoir units. This event plus unfortunate timing of Late Tertiary (Miocene) structural reactivation severely downgrade the petroleum potential of the basin. Paleogeography and Climate: Different PerspectivesEarly Jurassic paleogeography, portrayed by Christopher Scotese (Figure 3.6), shows the location of Takutu Basin. Interpretations of Early Jurassic paleogeography are shown in Figure 3.7, from Ron Blakey, and in Figure 3.8, from Robertson Research. These maps all show Takutu Basin to be intracontinental. The climate in the interior of Pangea, including the Takutu Basin area, was very hot and arid during source-rock deposition in Early to Mid Jurassic time (Figure 3.9).
Reduced TOC levels in two Takutu Basin wells (Turantsink 1 and Lethem 1) are due, at least in part, to over-maturation (gas window) of lacustrine shale source rocks of the Pirara and Manari formations (Figure 3.10). Type I/II kerogen is evident in early mature to mature Karanambo and Lethem samples (Figure 3.11). Low HI numbers inTurantsink 1 are due to overmaturity of the source interval. Overmaturity (Figure 3.12) and degradation of rich oil-prone lacustrine shale was caused by intense Miocene hydrothermal activity along south graben boundary faults.
The post-drill interpretation, based on the Turantsink 1 (Figure 3.13), is that the petroleum system was destroyed; this is based on the following:
Conclusions
ReferencesBlakey, Ron, 2004, Paleogeography through Geologic Time (http://jan.ucc.nau.edu/~rcb7/global_history.html): Northern Arizona University. Crawford, F.D., C.E. Szelewski, and G.D. Alvey, 1985, Geology and exploration in the Takutu graben of Guyana and Brazil: Journal Petroleum Geology, v. 8, p. 5-36. Robertson Research, Phanerozoic Paleogeographic Reconstructions and Major Source Rocks of the World. Scotese, Christopher, 2004, The Paleomap Project (http://www.scotese.com). |
