Mark Tingay

Origin of overpressure and pore-pressure prediction in the Baram province, Brunei

Accurate pore-pressure prediction is critical in hydrocarbon exploration and is especially important in the rapidly deposited Tertiary Baram Delta province where all economic fields exhibit overpressures, commonly of high magnitude and with narrow transition zones. A pore-pressure database was compiled using wireline formation interval tests, drillstem tests, and mud weights from 157 wells in 61 fields throughout Brunei. Overpressures are observed in 54 fields both in the inner-shelf deltaic sequences and in the underlying prodelta shales. Porosity vs. vertical effective stress plots from 31 fields reveal that overpressures are primarily generated by disequilibrium compaction in the prodelta shales but have been generated by fluid expansion in the inner-shelf deltaic sequences. However, the geology of Brunei precludes overpressures in the inner-shelf deltaics being generated by any conventional fluid expansion mechanism (e.g., kerogen-to-gas maturation), and we propose that these overpressures have been vertically transferred into reservoir units, via faults, from the prodelta shales. Sediments overpressured by disequilibrium compaction exhibit different physical properties to those overpressured by vertical transfer, and hence, different pore-pressure prediction strategies need to be applied in the prodelta shales and inner-shelf deltaic sequences. Sonic and density log data detect overpressures generated by disequilibrium compaction, and pore pressures are accurately predicted using an Eaton exponent of 3.0. Sonic log data detect vertically transferred overpressures even in the absence of a porosity anomaly, and pore pressures are reasonably predicted using an Eaton exponent of 6.5.

Present-day stress orientation in Brunei: a snapshot of ‘prograding tectonics’ in a Tertiary delta

The Baram Delta province of NW Borneo is unusual when compared with most other Tertiary deltas, as it has built up upon an active margin. Hence, structures observed in the Baram Delta province are the result of both margin-parallel gravity-driven deltaic tectonics and approximately margin-normal transpressive tectonics associated with the active margin. Image and dipmeter logs have been examined for breakouts and drilling-induced tensile fractures (DITFs) in 47 wells throughout Brunei. Breakouts and DITFs observed in 19 wells suggest that the maximum horizontal stress is oriented margin-normal (NW–SE) in the proximal parts of the basin and margin-parallel (NE–SW) in the outer shelf region. The margin-parallel outer shelf stress field is interpreted as a local ‘deltaic’ stress field caused by the shape of the clastic wedge. The margin-normal maximum horizontal stress in the inner shelf is interpreted to reflect basement stresses associated with the active margin. However, the maximum horizontal stress in the inner shelf is approximately perpendicular to the strike of Miocene–Pliocene normal growth faults, suggesting that maximum horizontal stress in the inner shelf has rotated from margin-parallel (‘deltaic’) to margin-normal (‘basement-associated’) over time. Hence, approximately the same stress rotation has occurred over time in the inner shelf as is currently observed spatially from the outer to inner shelf. The spatial and temporal stress rotations in Brunei are thus interpreted to be the result of ‘deltaic’ and ‘basement-associated’ tectonic regimes that are ‘prograding’ basin-wards. The proximity of the active margin has resulted in progressive uplift and inversion of the hinterland that has ‘forced’ the delta system to prograde rapidly. The zone of active deltaic growth faulting (and margin-parallel maximum horizontal stress) has shifted basin-wards (‘prograded’) as the delta system has rapidly prograded across the shelf. After uplift and delta progradation, the old growth faults of the inner shelf ceased being active and have then been successively reactivated by a similarly ‘prograding’ margin-normal inversion front.

Present-day stress and neotectonics of Brunei: Implications for petroleum exploration and production

The present-day state of stress in Tertiary deltas is poorly understood but vital for a range of applications such as wellbore stability and fracture stimulation. The Tertiary Baram Delta province, Brunei, exhibits a range of contemporary stress values that reflect the competing influence of the northwest Borneo active margin (situated underneath the basin) and local stresses generated within the delta. Vertical stress (v) gradients at 1500-m (4921-ft) depth range from 18.3 MPa/km (0.81 psi/ft) at the shelf edge to 24.3 MPa/km (1.07 psi/ft) in the hinterland, indicating a range in the shallow bulk density across the delta of 2.07–2.48 g/cm3. The maximum horizontal stress (Hmax) orientation rotates from margin parallel (northeast–southwest; deltaic) in the outer shelf to margin normal (northwest–southeast; basement associated) in the inner shelf. Minimum horizontal stress (hmin) gradients in normally pressured sequences range from 13.8 to 17.0 MPa/km (0.61–0.75 psi/ft) with higher gradients observed in older parts of the basin. The variation in contemporary stress across the basin reveals a delta system that is inverting and self-cannibalizing as the delta system rapidly progrades across the margin. The present-day stress in the delta system has implications for a range of exploration and production issues affecting Brunei. Underbalanced wells are more stable if deviated toward the hmin direction, whereas fracture stimulation in mature fields and tight reservoirs can be more easily conducted in wells deviated toward Hmax. Finally, faults near the shelf edge are optimally oriented for reactivation, and hence exploration targets in this region are at a high risk of fault seal breach.

Understanding tectonic stress in the oil patch The World Stress Map Project

Knowledge of the present-day tectonic stress is essential for numerous applications in petroleum exploration and production and in civil and mining engineering, such as improving the stability of boreholes and tunnels and enhancing petroleum production through natural or induced fractures. The World Stress Map (WSM) Project is a collaborative project between academia, industry, and government that is building a comprehensive global database of present-day stress information to better understand the state and sources of contemporary tectonic stress in the lithosphere (Figure 1).

Triggering of the Lusi mud eruption: Earthquake versus drilling initiation

ABSTRACTThe Lusi mud volcano in East Java has erupted unabated for almost 2 yr, fl ooding an area of 7 km 2 and displacing more than 25,000 people. Despite its disastrous impact, the mechanism for triggering the Lusi eruption remains highly controversial; two distinct mechanisms have been proposed. One hypothesis suggests that the eruption was triggered by the M w 6.3 earth-quake that struck Yogyakarta (250 km from Lusi) two days before the eruption. However, an examination of static and dynamic stress changes and stress transfer mechanisms indicates that the Yogyakarta earthquake was at least an order of magnitude too small to reactivate faults and open fl uid fl ow pathways under Lusi. An alternate theory suggests that Lusi was triggered by a blowout following drilling problems in the nearby Banjar Panji-1 well. Blowouts result from an inability to control pore fl uid intakes into the borehole and typically occur when the drilling win-dow (fracture pressure minus pore pressure) is approximately zero and when there is insuffi cient protective casing of the well bore. Pore and fracture pressure data from Banjar Panji-1 indicate that the well had a narrow drilling window of only 0–2.3 MPa. Furthermore, two planned casing points were skipped during drilling, resulting in 1742 m of unprotected borehole. The combina-tion of hazardously narrow drilling window and long uncased borehole would have made drill-ing problems in Banjar Panji-1 diffi cult to control, placing the well at high risk of blowing out. Furthermore, well-bore pressures following drilling problems in Banjar Panji-1 reached magni-tudes in excess of the fracture pressure and thus were suffi cient to create fl uid fl ow pathways in the subsurface. Therefore, we suggest that no viable method is known by which the Yogyakarta earthquake could have triggered the mudfl ow and that a blowout in the Banjar Panji-1 well was the most likely mechanism for triggering the Lusi eruption.Keywords:

‘Vertically transferred’ overpressures in Brunei: Evidence for a new mechanism for the formation of high-magnitude overpressure

Overpressures in sedimentary basins are commonly assumed to be the result of two distinct and separate mechanisms: disequilibrium compaction and fluid expansion. However, the potential for overpressures to be redistributed or transferred to other pressure compartments over time has been seldom considered and rarely demonstrated. Pore-pressure data and velocity-effective stress plots from 61 fields across the Baram Delta province of Brunei (northwest Borneo) reveal that two different types of overpressure occur in stratigraphically defined sections: the basal pro-delta shales contain overpressures generated by disequilibrium compaction, whereas the overlying sand/shale deltaic sequence contains overpressures that appear to be generated by fluid expansion. However, the geology of the deltaic sequences and high magnitude of the pore pressures precludes the overpressures in the deltaic sequences being generated by any conventional fluid expansion mechanism, such as kerogen-to-gas maturation or clay diagenesis. The fluid expansion overpressures are located in fields that were inverted during the Pliocene, an event that resulted in large-scale fluid migration from the pro-delta shales into the deltaic sequences, including charging of the numerous oil fields in the inner shelf. Hence, we propose that the overpressures in Brunei provide the first evidence for a new overpressuring mechanism whereby overpressured fluids have been “vertically transferred” from the pro-delta shales into the deltaic sequences during basin inversion. Furthermore, vertical transfer may be a mechanism for explaining overpressures observed in basins that have been recently uplifted or inverted.

Evidence for overpressure generation by kerogen-to-gas maturation in the northern Malay Basin

Gas generation is a commonly hypothesized mechanism for the development of high-magnitude overpressure. However, overpressures developed by gas generation have been rarely measured in situ, with the main evidence for such overpressures coming from source rock microfractures, the physical necessity of overpressures for primary migration, laboratory experiments, and numerical modeling. Indeed, previous in-situ observations suggest that gas generation only creates highly localized overpressures within rich source rocks. Pore-fluid pressure data and sonic velocity–vertical effective stress plots from 30 wells reveal that overpressures in the northern Malay Basin are primarily generated by fluid expansion and are located basinwide within the Miocene 2A, 2B, and 2C source rock formations. The overpressures are predominantly associated with gas sampled in more than 83% of overpressure measurements and have a sonic-density response consistent with gas generation. The association of fluid expansion overpressures with gas, combined with the sonic-density response to overpressure and a regional geology that precludes other overpressuring mechanisms, provides convincing in-situ evidence for basinwide gas generation overpressuring. Overpressure magnitude analysis suggests that gas generation accounts for approximately one-half to two-thirds of the measured excess pore pressure in the region, with the remainder being generated by coincident disequilibrium compaction. Thus, the data herein suggest that gas generation, if acting in isolation, is producing a maximum pressure gradient of 15.3 MPa/km (0.676 psi/ft) and not lithostatic magnitudes as commonly hypothesized. The gas generation overpressures in this article are not associated with a significant porosity anomaly and represent a major drilling hazard, with traditional pore-pressure prediction techniques underestimating pressure gradients by 2.3 ± 1.5 MPa/km (0.1 ± 0.07 psi/ft).

Pore pressure/stress coupling in Brunei Darussalam - implications for shale injection

Abstract Shale dykes, diapirs and mud volcanoes are common in the onshore and offshore regions of Brunei Darussalam. Outcrop examples show that shale has intruded along both faults and tensile fractures. Conventional models of overpressure-induced brittle failure assume that pore pressure and total stresses are independent of one another. However, data worldwide and from Brunei show that changes in pore pressure are coupled with changes in total minimum horizontal stress. The pore pressure/stress-coupling ratio (Δσh/ΔPp) describes the rate of change of minimum horizontal stress magnitude with changing pore pressure. Minimum horizontal stress measurements for a major offshore field where undepleted pore pressures range from normal to highly overpressured show a pore pressure/stress-coupling ratio of 0.59. As a consequence of pore pressure/stress coupling, rocks can sustain a greater increase in pore pressure prior to failure than predicted by the prevailing values of pore pressure and stress. Pore pressure/stress-coupling may favour the formation of tensile fractures with increasing pore pressure rather than reactivation of pre-existing faults. Anthropogenically-induced tensile fracturing in offshore Brunei supports this hypothesis.

Relationship between structural style, overpressures, and modern stress, Baram Delta Province, northwest Borneo

Christopher K. Morley, Mark Tingay and Richard Hillis Australian School of Petroleum, University of Adelaide, Adelaide, Australia Rosalind King

Structural controls on mud volcano vent distributions: examples from Azerbaijan and Lusi, east Java

Abstract: Structural mapping, nearest neighbour and two-point azimuth statistical analysis of mud volcano vent distributions from nine examples in Azerbaijan and the Lusi mud volcano in east Java are described. Distributions are non-random, forming alignments subparallel to faults within anticlines, ring faults, conjugate faults and detachment faults; this finding confirms a spatial relationship and supports a model for subsurface flow along these features as well as showing fractionation at depth. As fracture and fault orientations are related to structures such as anticlines and the in situ stress state they are therefore predictable. We use vent distributions in Azerbaijan, where the structural geology is well constrained, to propose what controls the distribution of 169 vents at the Lusi mud volcano. This mud volcano system shows evidence for initial eruptions along a NE–SW trend, parallel to the Watukosek fault, changing to eruptions that follow east–west trends, subparallel to regional fold axes. Our analysis indicates that regions east and west of the Lusi mud volcano are more likely to be affected by new vents than those to the north and south, owing to probable onset of elongate caldera collapse within a 10 km diameter of the central vent.