AAPG/GSTT HEDBERG CONFERENCE

“Mobile Shale Basins – Genesis, Evolution and Hydrocarbon Systems”

June 5-7, 2006 — Port-of-Spain, Trinidad and Tobago

Overpressure and fluid migration in the evolution of the Baram and Champion deltas, Brunei: impact on hydrocarbon systems.

Pieter Van Rensbergen¹, Mark Tingay², Christopher Morley³, John Warren⁴, Lai Quoc Lap⁴,

Ian Cartwright⁵, Herman Darman⁶

¹Ghent University, Belgium

²Karlsruhe University, Germany

³PTT Exploration & Production, Thailand

⁴University of Brunei Darussalam, Brunei

⁵Monash University, Australia

⁶Brunei Shell Petroleum, Brunei

NW Borneo’s largest oil fields are located within the Miocene Champion and Pliocene Baram delta systems, onshore and offshore Brunei Darussalam. Hydrocarbon fields occur at inversion anticlines within the Miocene Champion delta system and in total have production exceeding a billion barrels of oil. Other large fields occur at the outer shelf and slope of the Champion and Baram delta system and there is current major exploration focus on the deepwater fold and thrust belt at the delta toe. The geology of the delta systems is well known but the hydrocarbon charging mechanism remains elusive. The Champion-Baram delta system is a compact system, smaller in scale than many other mobile shale basins – therefore it makes an integrated approach possible.

In this study we discuss the hydrocarbon system offshore Brunei and why major hydrocarbon fields are so abundant and geographically concentrated. Offshore Brunei has experienced a perfect combination of active, detachment tectonics and rapid sedimentation for the development of a major petroleum system. Detachment tectonics are important in the delta evolution and for developing fluid migration pathways, however, the role of shale mobility in these process is less well understood.

Structure

The structural grain of the Baram and Champion deltas consists primarily of NE-SW striking back-to-back counter-regional and regional growth faults. In some areas the regional growth faults are dominant (e.g. Outer Shelf growth fault, Baram delta), while in other regions the counter-regional growth faults are better developed (e.g. Perdana fault, Champion delta). In the inner shelf and onshore part of the delta systems, these back-to-back faults were inverted during a Pliocene compressional period. Today, these resultant anticlines host the larger (giant) oil fields. The outer shelf is characterized by active growth faults (mainly counter-regional systems at the eastern Champion delta and regional systems at the western Baram system). The slope is a typical fold and thrust belt with numerous fluid escape features.

Overpressure

Overpressures throughout Brunei commonly reach very high magnitudes (close to lithostatic) in most of the fields. Overpressures in the outer shelf become progressively shallower passing offshore and lie almost exclusively within or near the top of the low permeability pro-delta shale. In this case the overpressures are associated with low sonic velocities and a gradual increasing effective vertical stress path. These overpressure occurrences can be attributed to disequilibrium compaction. In the inner shelf, high and shallow overpressures occur within shale-cored inversion anticlines. The depth to the top of overpressure is highly variable and pore pressures change rapidly across thin shale units and faults, indicating a high degree of compartmentalization. These overpressures are associated with high sonic velocities and sharp decrease in effective vertical stress. On porosity/effective vertical stress plots, the presence of numerous data points that plot off the loading curve indicate these overpressures were generated by inflation due to lateral or vertical fluid transfer rather than being the in situ products of disequilibrium compaction. Inflationary overpressures are also observed in inner shelf hydrocarbon reservoirs located in normal growth fault settings at the Baram delta.

Source rock

The hydrocarbon source rock is believed to be pro-delta shale, typically containing 2-3% predominantly allochtonous land-derived organic carbon of type III gas prone kerogen (Sandal 1996, Curiale et al 2000). Prodelta shale accumulates thickly in counter-regional growth faults, where pro-delta turbidite lobes are trapped against the faults’ footwall. In this setting, source rock material is trapped at the landward margin of back-to-back fault systems. Oil generation peaked in the Pliocene and the main phase of oil generation occurs at 3000 m depth, gas generation occurs at 3500 m to 4500 m – 5500 m depth. Below 3 sec (~ 4500 m depth) seismic blanking is widespread within the counter-regional depocentres, both in the outer shelf and at the landward margin of the inversion anticlines in the inner shelf. This blanking zone is found to be the root of a buried mud volcano system and was attributed to overpressure generation, probably related to kerogen-to-gas transformation.

Fluid migration

The counter-regional growth fault depocentres are possibly the source of inflation overpressures that have charged the inverted anticlines and the Baram fields. These depocentres accentuate the extra-ordinary structure of the inner shelf anticlines, in which the source rock and reservoir sands are superimposed in the inverted back-to-back fault system. The anticlines have a clear asymmetry with a mud-rich landward flank and a sand-rich basinward flank. Lateral up dip migration from adjoining counter-regional growth fault depocentres contributed to the charging of the outer compartments, in addition to vertical migration along faults and fractures. The Baram fields outside of the Champion delta realm were possibly charged by seismic pumping and lateral fluid migration from the Champion delta source rocks along faults during the Pliocene inversion period.

Isotopic analysis of carbonate cements (nodules and cemented layers) tied to seismic and organic geochemical signatures across various oilfields or from the present deep seafloor sediments of offshore Brunei clearly illustrates a structural focus to fluid migration. This is so both in Miocene shelf sand reservoirs and in modern offshore deepwater muds. At least since the Miocene carbonate cementation in Brunei has been an ongoing process from initial siderite/dolomite deposition (immediately beneath the seafloor) well into the burial realm. Most of the early cement signatures, expressed as carbon and oxygen isotope values, are typical of nodules forming in a widespread sulphate reduction zone located just below the seafloor. But sometimes the nodule signatures in oilfield cements show values that are characteristic of methanogenic seafloor seeps worldwide. These intervals tend occur in Miocene shelf sediments near what are interpreted as former growth fault -seafloor intersections. Cements continue to form well into the burial realm where some ferroan dolomites define cement layers located at present or former gas-water contacts. FMI analysis shows that these cemented layers in reservoir sands are correlatable within a field and that they tend to thicken toward feeder faults. Later stages of these cements form as saddle dolomites with minor metal sulphide precipitates. Integration of the chromatography of organics in modern seafloor sediments (collected for analysis as frozen samples from piston cores to 3 metres depth below the seafloor surface) with isotopic signatures of associated carbonate nodules and tied to 3 D seismic show that seeps in the deep water muds of present-day offshore Brunei are tied to faults, eroded anticlinal crests atop ramping thrusts and zones where mud volcanoes intersect the seafloor.

Present-day stress orientations determined from borehole breakouts in the inner shelf still reflect the Pliocene compressional event. However, the vertical stress magnitude across the delta decreases distally due to variable uplift of the hinterland and undercompaction of the prodelta shales. At the delta toe the vertical stress magnitude decreases relative to the minimum horizontal stress magnitude. No modern stresses have been determined for the delta toe, however the presence of growing anticlines suggests that a present-day compressional stress regime (SHmax>Shmin>Sv) exists at the delta toe. A compressional stress regime may also possibly be present in the undercompacted prodelta shale at the base of the delta system, near the detachment zone. The effective vertical stress magnitude is very low in undercompacted shales and may be even lower in mature source rock areas if overpressuring is enhanced by kerogen-to-gas maturation. Hence, hydraulic fractures near the base of the delta system may be horizontal, promoting basinward fluid flow. The increase of horizontal pressure during the Pliocene compression event will have theoretically increased this effect and caused important basinwards pumping of overpressured fluids and hydrocarbons into the outer Baram fields, and possibly also into the fold and thrust belt at the delta toe. This fluid migration mechanism is supported by observations from seismic reflection data across the outer shelf to slope which shows fluid escape features, which can be accompanied by localized shallow diapirism. On the other hand, vertical migration from hydrocarbon generation from basinal sediments cannot be ruled out and both processes may act together.

Conclusion

This integrated research into the offshore regions of Brunei suggests important indications of lateral fluid flow from below the Miocene delta deposits towards the present-day toe and thrust belt. We know that undercompacted shale has been widely mobilized as dyke systems, vertical pipes and mud volcanoes from outcrop evidence in the Jerudong anticline and seismic studies in offshore fields. However, we also find that the amount of mobile shale has been largely overestimated in previous studies due to poor seismic resolution. The occurrence of mobile shale in the slope system also appears to have been overestimated, where fluid injection features have been confused with zones of mobile shale emplacement.