EXPLORATION World class Devonian potential seen in eastern Madre de Dios basin

Ken E. Peters John B. Wagner
Mobil Technology Co.
Dallas
Daniel G. CarpenterMobil New Business Development, Americas
Dallas
Keith T. Conrad
Mobil Exploration Norway
Stavanger

The Madre de Dios basin in northern Bolivia contains thick, laterally extensive, organic-rich Upper Devonian source rocks that reached the oil-generative stage of thermal maturity after trap and seal formation. Despite these facts, less than one dozen exploration wells have been drilled in the Madre de Dios basin, and no significant reserves have been discovered.

The basin covers a large area nearly one fourth the size of the prolific Western Canada basin (~16 versus 70 million hectares, respectively). Many exploration plays in both basins have focused on Paleozoic source rocks and stratigraphic traps.

The Upper Devonian rocks in the Madre de Dios basin are among the richest petroleum source rocks in the world. These world class source rocks are richer than the Upper Jurassic source rocks in Central Saudi Arabia or the North Sea. The Upper Devonian source rocks are thermally mature thoughout much of the basin. Unlike the Madre de Dios basin, the Western Canada basin contains more than 50,000 wells, 10 giant fields, and about 25 billion bbl of oil equivalent.

Mobil geoscientists conducted a regional geological, geophysical, and geochemical study of the Madre de Dios basin. The work reported here was designed to assess the distribution, richness, depositional environment, and thermal maturity of Devonian source rocks. It is supported by data from over 3,000 m of continuous slimhole core in two of the five Mobil wells in the basin. Source potential also exists in Cretaceous, Mississippian, and Permian intervals. The results of this study have important implications for future exploration in Bolivia and Peru.

Geology, stratigraphy

The Subandean hydrocarbon megatrend contains many basins with proven reserves (Fig. 1 [120296 bytes]). The Madre de Dios block in Bolivia is located on the gentle dip slope of the tectonically active sub-Andean foreland basin. Despite its favorable location along the megatrend, major accumulations of hydrocarbons have yet to be discovered in the Madre de Dios area.

The Madre de Dios basin contains a sequence of Paleozoic to Tertiary sediments up to 9 km thick, overlying Precambrian basement (Fig. 2 [79018 bytes]). During the Paleozoic, sediments were deposited in an intracratonic setting that only began to evolve toward the present-day foreland basin during the Mesozoic. The Paleozoic intracratonic phase accounts for deposition of the principal petroleum source and reservoir rocks, while a late Oligocene-Recent Andean tectonic phase resulted in burial, source rock maturation, and migration. Tectonic events are referenced to orogenic activity in Fig. 2.

An important Cretaceous angular unconformity separates the passive margin from the overlying subduction-collision sequences in the Bolivian foreland (Figs. 3 [50721 bytes]and 4 [88580 bytes]). This unconformity is interpreted to correlate with the onset of plate convergence where oceanic crust was subducted beneath continental crust. Back-arc thrust belts are created when the rate of convergence exceeds that of subduction. Foreland basins associated with back-arc thrust belts subside and migrate toward the craton as the thrustbelts encroach and create loading, generating a peripheral bulge. With continued convergence, the peripheral bulge continues to extend the unconformity in front of the encroaching thrust belt. Marine shales in asymmetric foreland basins commonly act as source rocks, seals, and sites of decollement. In our model, burial maturation of source beds in foreland basin depocenters is followed by migration of petroleum up-dip toward the gently sloping side of the basin.

The lowermost sedimentary sequence in the basin consists of Middle Ordovician to Late Silurian fluviodeltaic units capped by a thin, shallow marine carbonate (Fig. 2). This unit has good reservoir and seal potential, but may be isolated from the Upper Devonian source rocks by Lower Devonian marine shelfal shales with poor source potential (Fig. 3). The seismic character and tectonic setting of the overlying Devonian rocks (Fig. 4) are remarkably similar to those of the Upper Jurassic Bazhenov source rocks in Western Siberia. Both source rocks were deposited during widespread marine transgressions and are overlain by prograding deltaic systems.

The Middle Devonian consists of well developed shallow marine sandstones that were deposited during falling sea level and were subsequently reworked during a marine transgression. These shoreface sandstones are potential reservoir targets because they are intercalated with, and overlain by Upper Devonian source rocks, which may also act as a seal.

The petroleum-generative potential of the Upper Devonian Tomachi formation shales and their distribution suggest that the Madre de Dios area has the potential for large accumulations. The Upper Devonian source rocks are overlain by two Upper Devonian and Lower Carboniferous deltaic sequences with excellent reservoir quality. These subaerial to subaqueous lower delta plain deposits consist of distributary channels and distributary mouth bars that show northeast to southwest basinward progradational geometries.¹ However, because of the regional dip and common lack of structural closure (Figs. 3-4), volumetrically important hydrocarbon accumulations would require stratigraphic traps (e.g., sandstone pinchouts).

Following deposition of the Late Devonian fluvial/ deltaic sequences, a major climatic change occurred in response to continental translation. A depositional hiatus of about 50 Ma marks this climatic change from cooler, wetter Devonian deltaic settings to warmer, more arid Carboniferous to Early Permian coastal sabkhas, aeolian dunes, and shallow marine settings.

Geochemistry

Seeps of crude oil and gas are common along the foredeep/thrust belt on the southwestern edge of the Madre de Dios basin and suggest that significant amounts of hydrocarbons were generated (Fig. 5 [51274 bytes]). However, seeps are rare updip and northeast toward the Brazilian Shield. All five wells drilled by Mobil in the basin contain shows in the Devonian to Lower Permian rocks but were noncommercial.

The Pando X-1, Pando X-2, Alianza X-1, Manuripi X-1, and Neuva Victoria X-1 wells were drilled approximately east-west along basinal strike in Mobil's Madre de Dios block. In this article, we focus on the Mobil/Occidental Pando X-1 well, where low-sulfur (0.14 wt %), 32° gravity oil was produced from Upper Devonian sandstones.

The geochemical log² for continuous core from the Pando X-1 well shows Devonian organic-rich shales in the range 1,350 to 1,590 m that contain up to 16 wt % total organic carbon (Fig. 6 [97703 bytes]) near 1,570 m. Organic matter quality as measured by Rock-Eval pyrolysis generally increases with depth in this interval, reaching maximum hydrogen index values near 600 mg hydrocarbon/g TOC at about 1,510 m. High hydrogen ( 400 mg hydrocarbon/g TOC) and low oxygen indices (< 20 mg co2/g toc) for organic matter in most of these rocks indicate oil-prone marine type i/ii kerogen. gamma ray response shows two maxima at about 1,510 and 1,570 m in the shales. lithofacies analysis indicates that the rocks near 1,510 m best correspond to the maximum flooding surface associated with a regional marine transgression. these organic-rich, oil-prone shales are thermally immature based on a combination of thermal alteration index (tai), tmax, vitrinite reflectance (ro), and production index values (pi).

Biomarker and stable carbon isotope analyses³ for selected Devonian rocks and a crude oil from 1,266 m in the Pando X-1 well confirmed an active petroleum system. For example, sterane distributions in the Pando X-1 oil correlate best with the Upper Devonian source-rock extract from 1,510 m (Fig. 7 [101809 bytes]).

Palynofacies analysis of the three main shale intervals supports the geochemical results (Fig. 6). Clay-rich Upper Devonian-Lower Carboniferous prodelta shales from 1,139-95 m in the Pando X-1 well contain amorphous kerogen types that indicate oxic, nearshore depositional conditions. Structured organic matter in this interval is dominated by trilete spores and other terrigenous, higher-plant detritus.

The organic-rich Upper Devonian prodelta to marine shelf shales from 1,231-1,587 m contain amorphous kerogen types that indicate anoxic depositional conditions. Prasinophyte algal remains are abundant, especially just below the maximum flooding surface near 1,510 m (MFS). Prasinophyte algae, including Tasmanites, are commonly associated with nutrient-rich marginal marine settings. We observed an increase in tricyclic terpanes relative to hopanes in Pando X-1 rock samples that contained elevated Prasinophyte algal remains (Fig. 6), consistent with published evidence that these compounds are markers of Tasmanites.⁴ The tricyclic terpane/hopane ratio for the crude oil from 1,266 m in the well suggests that it is most closely related to source rock near 1,510 m, which is consistent with the sterane and other source-related geochemical data.

Lower Devonian muddy shelf shales from 1,750-1,917 m show amorphous kerogen types with a transitional composition between the oxic and anoxic facies. Acritarchs dominate the structured organic matter in this interval, consistent with normal, open marine sedimentation. The Lower and Upper Devonian shales show similar seismic character (Fig. 4), but depositional conditions during the Late Devonian were optimal for source rock development.

The Upper Devonian interval in the Pando X-1 and X-2 wells is world class petroleum source rock based on the Source Potential Index (18 and 15 tons of hydrocarbon/sq m, respectively) calculated from Fig. 6. These SPI values are comparable to those for prolific Upper Jurassic source rocks in the North Sea and Central Saudi Arabia, and the Middle to Upper Cretaceous in the Middle Magdalena Valley of Colombia and the Maracaibo basin in Venezuela (15, 14, 16, and 27, respectively).⁵

Next: Devonian paleodepositional setting, thermal maturity and basin modeling, and conclusions.