Pyrobitumen occurrence and formation in a Cambro–Ordovician sandstone reservoir, Fahud Salt Basin, North Oman

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Abstract

The Cambro–Ordovician Barik Sandstone reservoirs in the Fahud Salt Basin in Oman contain bitumen which may fill up to 40% of the porosity. In well Jaleel-1, this bitumen was isolated (according to kerogen procedure) and typed by NMR, elemental analysis and density measurements. The isolated bitumen is characterized by: (1) a highly aromatic character (NMR 75% CAro, H/C atomic ratio: 0.65), (2) a very high sulphur content (4.2%) and (3) a relatively high density (1.3–1.4 g/cm3). The insolubility and the reflectivity of the bitumen (1.2% Vr) qualify it as a low mature pyrobitumen. The combination of Rock-Eval and density data was used to calculate the actual volume of the pyrobitumen in the rock, as a percentage of porosity. It was found that the pyrobitumen volume shows a negative correlation with total porosity, indicating that small pores are more invaded by bitumen than larger ones. Finally, closed system pyrolysis experiments, performed on oils with different NSO contents, indicate that an in situ oil with a very high content of NSO compounds is required to generate such large amounts of pyrobitumen in the pore system. These observations suggest that the precursor oil of the current pyrobitumen was a very heavy oil tentatively assumed to be the result of a severe biodegradation. Basin modeling shows that the reservoir was charged already in Devonian times. A major uplift brought the oil accumulation near the surface during the Carboniferous and a rather regular burial to the present day position (4500 m, 140°C) (Loosveld et al., 1996). This scenario, involving a residence time at shallow depth, strengthens the biodegradation hypothesis. The numerical modeling, which involves the IFP kinetic model for secondary oil cracking, suggests that pyrobitumen formation is a very recent event. Inclusion of pyrobitumen particles within quartz overgrowth, containing fluid inclusions, provides an upper temperature limit for the beginning of pyrobitumen formation which comforts the result of kinetic modelling.

Introduction

The current trend in hydrocarbon exploration to target deep, high temperature prospects has encouraged research on the thermal stability of petroleum. The onset of oil cracking is still a matter of debate but has been predicted for certain oils at temperatures as low as 140°C (Schenk and Horsfield, 1995). The hydrogen mass balance of oil cracking implies that apart from gas, a highly aromatic and insoluble carbonaceous residue, pyrobitumen, is generated. Because deposition of pyrobitumen may significantly deteriorate reservoir quality, bitumen plugging is recognized as a major exploration risk in many hydrocarbon provinces. This is the case of North Oman where bitumen is often identified on routine microscopical examination of thin sections and where several exploration wells have penetrated reservoirs where the porosity is plugged to such an extent that even gas production is impaired.

The purpose of this paper is to investigate the nature and formation of the bitumen occurrences encountered in the Cambro–Ordovician fluvio-deltaic Barik Sandstone reservoirs in the Fahud Salt Basin in Oman.

Section snippets

Geological setting

The Jaleel prospect belongs to the Fahud basin which is located on the eastern flank of a north–south graben system. The Fahud basin is separated from the Ghaba salt basin, sited in the central graben itself, by the Makarem-Mabrouk High Fig. 1, Fig. 2. The graben system has been emplaced during Infracambrian time, and is coeval with the rifting phases associated with a sinistral motion along the Najd fault which crosses the Arabian plate along a NW–SE direction from Bani Ghayy basins in Saudi

Samples

Twenty-one plugs (JLL1 to JLL21) selected from the 11-m cored section (4560–4571 m) of the Jaleel 1 well were complemented by two cuttings samples respectively assigned to depth intervals: 4480–4560 m (JLLCUT1) and 4571–4636 m (JLLCUT2). As a consequence, the entire set of samples can be considered as representative of a 150-m thick interval of the reservoir. The core samples have been collected at spots where porosity data were available. The two cuttings samples have been separated into two

Experimental

Rock-Eval II pyrolysis has been performed on the whole set of samples using the reservoir mode (temperature heating ramp: 10°C/min). Besides the classical S1 peak (light thermovaporizable fraction), this specific mode allows to deconvolute the S2 peak into S2a and S2b peaks which can be respectively assigned to a heavy thermovaporizable fraction and to an actual pyrolyzable fraction (Trabelsi et al., 1994)(Fig. 4). The residual organic carbon is oxidized and the resulting CO2 is recorded as S4

Organic content

According to Fig. 5, the TOC of the 21 core samples ranges from about <0.01wt.% to 2.3 wt.%, although most values are between 0.3 wt.% and 1.2 wt.%, and the average value is 0.7 wt.%.

TOC content of bulk cuttings samples (based on 10 aliquots for each sample), although slightly lower than the core samples average, is comparable to this value: 0.5 wt.% (JLLCUT1) and 0.3 wt.% (JLLCUT2). The TOC content of the hand-picked lithological fractions from JLLCUT1 and JLLCUT2 ranges, respectively, from

Conclusion

Based on the available geological and geochemical information, the following scenario is proposed to explain the observed pyrobitumen occurrence in the Jaleel prospect (Fig. 12). Emplacement into the Barik sandstone member of an early charge of oil generated during the Ordovician by a Huqf source rock which was buried as a result of a rifting phase initiated in Infracambrian time. This charging episode was followed by a regional uplift affecting eastern Oman during late Paleozoic and bringing

Acknowledgements

We are indebted to J. Amthor, J. Borgomano, J. Terken, and N. Al-Ruwehy for the fruitful discussions. The used petroleum geology of North Oman is the result of the work of many PDO and Shell Geoscientists, who cannot all be mentioned here. We want to extend our appreciation to the comments made on the first manuscript by J.M. Gaulier, F. Roure and M. Vandenbroucke.

This paper is authorized by the Petroleum Development of Oman LLC and the Ministry of Petroleum and Minerals of Oman. The authors

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