Rosalind King

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.

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

Present-day stress orientations and tectonic provinces of the NW Borneo collisional margin

Received 23 September 2009; revised 17 May 2010; accepted 9 June 2010; published 16 October 2010. [1] Borehole failure observed on image and dipmeter logs from 55 petroleum wells across the NW Borneo collisional margin were used to determine maximum horizontal stress (sH) orientations; combined with seismic and outcrop data, they define seven tectonic provinces. The Baram Delta–Deepwater Fold‐Thrust Belt exhibits three tectonic provinces: its inner shelf inverted province (sH is NW‐SE, margin‐normal), its outer shelf extension province (sH is NE‐SW, margin‐parallel), and its slope to basin floor compression province (sH is NW‐SE, margin‐normal). In the inverted province, sH reflects inversion of deltaic normal faults. The sH orientations in the extension and compression provinces reflect deltaic gravitational tectonics. The shale and minibasin provinces have been recognized in offshore Sabah. In the shale province, sH is N010°E, which aligns around the boundary of a massif of mobile shale. Currently, no data are available to determine sH in the minibasin province. In the Balingian province, sH is ESE‐ WNW, reflecting ESE absolute Sunda plate motions due to the absence of a thick detachment seen elsewhere in NW Borneo. The Central Luconia province demonstrates poorly constrained and variable sH orientations. These seven provinces result from the heterogeneous structural and stratigraphic development of the NW Borneo margin and formed due to complex collisional tectonics and the varied distribution and thicknesses of stratigraphic packages.

Detrital zircon data reveal the origin of Australia's largest delta system

The Late Cretaceous Ceduna Delta is the largest deltaic system on the Australian continent, yet its source is unknown. Apatite fission-track data reveal widespread Late Cretaceous exhumation across the southern Australian margin. New detrital zircon analysis of 786 grains from the Gnarlyknots-1 well, which penetrated the offshore delta top, show that the upper part of the delta (Santonian–Maastrichtian) was sourced largely from recycled Permian to Early Cretaceous cover and underlying basement eroded from the margin, proximal to the basin. This challenges the widely accepted model involving distal provenance of >2000 km from the eastern margin of Australia. Supplementary material The 2D seismic reflection data, results for detrital zircon LA-ICP-MS and zircon fission-track analyses, including the LA-ICP-MS method, and a list of sample intervals and ages are available at www.geolsoc.org.uk/SUP18582.

Present-day stress and neotectonic provinces of the Baram Delta and deep-water fold–thrust belt

Abstract: We present the first present-day stress orientation measurements across a delta and deep-water, delta toe fold–thrust belt. Stress and neotectonic data for NW Borneo reveal three discrete neotectonic provinces: an inverted, inner shelf province (where present-day maximum horizontal stress, σH, is oriented margin-normal); an extensional, delta top province (σH is margin-parallel), and a compressional, delta toe province (σH is margin-normal). We interpret the delta top and delta toe data to reflect that compression in the fold–thrust belt is coupled to delta top extension and that the inverted, inner shelf province reflects the location of this system on an active margin.

In situ stresses and natural fractures in the Northern Perth Basin, Australia

Present-day stress orientations in the Northern Perth Basin have been inferred from borehole breakouts and drilling-induced tensile fractures observed on image logs from eight wells. Stress indicators from these wells give an east – west maximum horizontal stress orientation, consistent with stress-field modelling of the Indo-Australian Plate. Previous interpretations using dipmeter logs indicated anomalous north-directed maximum horizontal stress orientations. However, higher-quality image logs indicate a consistent maximum horizontal stress orientation, perpendicular to dominant north – south and northwest – southeast fault trends in the basin. Vertical stress was calculated from density logs at 21.5 MPa at 1 km depth. Minimum horizontal stress values, estimated from leak-off tests, range from 7.4 MPa at 0.4 km to 21.0 MPa at 0.8 km depth: the greatest values are in excess of the vertical stress. The maximum horizontal stress magnitude was constrained using the relationship between the minimum and maximum horizontal stresses; it ranges from 8.7 MPa at 0.4 km to 21.3 MPa at 1 km depth. These stress magnitudes and evidence of neotectonic reverse faulting indicate a transitional reverse fault to strike-slip fault-stress regime. Two natural fracture sets were interpreted from image logs: (i) a north- to northwest-striking set; and (ii) an east-striking set. The first set is parallel to adjacent north- to northwest-striking faults in the Northern Perth Basin. Several east-striking faults are evident in seismic data, and wells adjacent to east-striking faults exhibit the second east-striking set. Hence, natural fractures are subparallel to seismically resolved faults. Fractures optimally oriented to be critically stressed in the present-day stress regime were probably the cause of fluid losses during drilling. Pre-existing north- to northwest -striking faults that dip moderately have potential for reactivation within the present-day stress regime. Faults that strike north to northwest and have subvertical dips will not reactivate. The east-striking faults and fractures are not critically stressed for reactivation in the Northern Perth Basin.

Detrital zircon analysis of the southwest Indochina terrane, central Thailand: Unravelling the Indosinian orogeny

The Khao Khwang fold-and-thrust belt, central Thailand, developed within a basin that formed on the southwestern margin of the Indochina block. Because of limited geochronological and provenance constraints, the time of deposition, sediment source location, and tectonic significance of the basin have been uncertain. Here, we present 837 U-Pb detrital zircon ages and 271 Hf isotope in situ analyses from Permian−Triassic clastic units within the Khao Khwang fold-and-thrust belt in order to constrain the provenance, maximum depositional ages, and depositional environment of the southwestern margin of the Indochina terrane through the late Paleozoic to early Mesozoic. The key lithological units, the Sap Bon, Pang Asok, and Nong Pong Formations, are part of the Saraburi Group and have detrital age spectra spanning from Late Triassic to Paleoarchean. The entire data set has a common age peak at ca. 450 Ma, and all samples contain grains with ages of 0.2−0.3, 0.4−0.6, 1.0−1.3, 1.7−1.8, and 2.2−2.7 Ga. A few grains predate 3.0 Ga. Multidimensional scaling analysis of detrital zircon ages from throughout SE Asia demonstrates that the age spectra of the siliciclastic units of the Saraburi Group resemble those of Permian−Triassic detritus found elsewhere in the Khorat Plateau and throughout Vietnam and southeast China, implying that these areas shared similar sources. These sources may have been the, now largely covered, Indochina basement, and/or contiguous continental crust in terranes already amalgamated to Indochina at that time. Detrital zircons as young as 205 ± 6 Ma show that some formations of the Saraburi Group, previously considered to be of Middle−Late Permian age, are no older than Late Triassic. We propose a depositional model for the region of a Permian rift or passive-margin setting that evolved into piggyback and foredeep basins during an extended period of folding and thrusting in the Triassic.

Determination of the tectonic evolution from fractures, faults, and calcite twins on the southwestern margin of the Indochina Block

In polyphase tectonic zones, integrating a study of fault and fracture with calcite twin analysis can determine the evolving paleostress magnitudes and principle stress directions that affected the area. This paper presents the results of the analyses of fractures, striated faults, and calcite twins collected within the Khao Khwang Fold-Thrust Belt in central Thailand (SE Asia). Here we attempt to reconstruct the orientation of the principal stresses that developed during the tectonic evolution of this highly deformed, polyphase orogen. Tectonic data were collected in the Permian carbonates of the Khao Khad Formation of the Saraburi Group, and five successive tectonic stages are determined that are interpreted to have developed before, during, and after, the Triassic Indosinian Orogeny. The first three stages predate the main deformation event: the first stage is interpreted as a pre-Indosinian N-S extensional stage, the second stage described a N-S strike-slip and compressional regime, largely perpendicular to the fold axes of the main structures, while the third stage is associated with an E-W compressional strike-slip phase. A further two stages took place after, or during, the main folding event and correspond to N-S compression and to an E-W composite strike-slip/contractional stage, the latter which is interpreted to represent Cenozoic deformation related to the India-Asia collision.

Remote sensing of subsurface fractures in the Otway Basin, South Australia

Naturally occurring fractures were remotely detected in a 3-D seismic volume from the Penola Trough in South Australias Otway Basin and validated through an integrated approach. Identified in image logs are 508 fractures and 523 stress indicators, showing maximum horizontal stress orientation in the Penola Trough is 127°N. Two fracture types were identified: (1) 268 electrically conductive (potentially open to fluid flow) fractures with mean NW-SE strikes and (2) 239 electrically resistive (closed to fluid flow) fractures with mean E-W strikes. Core from Jacaranda Ridge-1 shows that open fractures are rarer than what image logs indicate, due to the presence of fracture-filling siderite, an electrically conductive cement which may cause fractures to appear hydraulically conductive in image logs. The majority of fractures detected is favorably oriented for reactivation under in situ stresses, although it is demonstrated that fracture fills primarily control which fractures are open. Seismic attributes calculated from the 3-D Balnaves/Haselgrove survey are mapped to the Pretty Hill Formation to enhance observations of structural fabrics, showing linear discontinuities likely representing faults and fractures. Discontinuity orientations are consistent with natural fracture orientations identified in image logs, striking E-W and NW-SE, limited to zones around larger faults. However, it is unlikely that a large proportion of these fractures are open given observations of core and image logs, limiting possible fracture connectivity and therefore significant secondary permeability in the Penola Trough. The integrated methodology presented herein provides an effective workflow for remote detection of subsurface fractures and determining if electrically conductive fractures are also hydraulically conductive.

Geomechanical modelling of fault reactivation in the Ceduna Sub-basin, Bight Basin, Australia

Abstract The Ceduna Sub-basin is located within the Bight Basin on the Australian southern margin. Recent structural analysis using newly acquired two-dimensional (2D) and three-dimensional (3D) seismic data demonstrates two Late Cretaceous delta–deepwater fold–thrust belts (DDWFTBs), which are overlain by Cenozoic sediments. The present-day normal fault stress regime identified in the Bight Basin indicates that the maximum horizontal stress (SHmax) is margin parallel; Andersonain faulting theory therefore suggests the delta-top extensional faults are oriented favourably for reactivation. A breached hydrocarbon trap encountered in the Jerboa-1 well demonstrates this fault reactivation. Faults interpreted from 3D seismic data were modelled using the Poly3D© geomechanical code to determine the risk of reactivation. Results indicate delta-top extensional faults that dip 40–70° are at moderate–high risk of reactivation, while variations in the orientation of the fault planes results in an increased risk of reactivation. Two pulses of inversion are identified in the Ceduna Sub-basin and correlate with the onset of rifting and fault reactivation in the Santonian. We propose a ridge-push mechanism for this stress which selectively reactivates extensional faults on the delta-top, forming inversion anticlines that are prospective for hydrocarbon exploration.