AAPG/GSTT HEDBERG CONFERENCE
“Mobile Shale Basins – Genesis, Evolution and Hydrocarbon Systems”
and fluid migration in the evolution of the Baram and Champion deltas,
Pieter Van Rensbergen1, Mark Tingay2, Christopher Morley3, John Warren4, Lai Quoc Lap4,
Ian Cartwright5, Herman Darman6
Exploration & Production,
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
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.
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.
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
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.
This integrated research into the
offshore regions of
AAPG Search and Discovery Article #90057©2006 AAPG/GSTT Hedberg Conference, Port of Spain, Trinidad & Tobago