VULCAN SUB-BASIN FAULT STYLES — IMPLICATIONS FOR HYDROCARBON MIGRATION AND ENTRAPMENT

1992 ◽  
Vol 32 (1) ◽  
pp. 138 ◽  
Author(s):  
E.P. Woods
Keyword(s):  
Late Stage ◽  
Structural Complexity ◽  
Strike Slip ◽  
Normal Faults ◽  
Third Order ◽  
Hydrocarbon Migration ◽  
Structural Domains ◽  
The North ◽  
Hydrocarbon Traps ◽  
Timor Sea

Several structural domains are recognised within the Vulcan Sub-basin, Timor Sea. These domains developed during the Jurassic rifting phase and are separated by major transfer zones which trend in a northwest-southeast direction. Within each domain are frequent third order transfers which sub-divide the main northeast trending fault blocks into numerous compartments. These enable structural hydrocarbon traps to be formed, despite a predominant regional dip. They also affect migration pathways.Jurassic fault styles include detached rotational blocks, salt-associated features, tilted fault blocks and 'hourglass' horsts and grabens. These generally have a northeast-southwest orientation. The transfer faulting complicates these features and forms zones of structural complexity with associated poor seismic data quality. A separate fault episode in the north of the sub-basin during the Tithonian resulted in an east-west fault set overprinting the earlier structuring.Intra-Cretaceous fault movement is also recognised and has an important role in early hydrocarbon entrapment.Structural reactivation during the Late Miocene/Early Pliocene of the earlier fault sets modified the geometry of many existing traps. Numerous new traps may also have formed as a result of this tectonism. In many places the resulting geometry is complex, particularly where the younger fault orientation is at an angle to the main Oxfordian fault set. The late-stage movement is primarily extensional, manifested by predominantly normal faults; overall, however, a varying component of strike slip is likely. A divergent strike-slip zone is recognised at the southwest end of the Cartier Trough.The effects of the late stage tectonism tend to mask the seismic expression of Mesozoic hydrocarbon traps resulting in many wells being drilled off-structure at the target horizon. An understanding of the deeper structuring should result in further discoveries in this prospective basin.

THE ELANG OIL DISCOVERY ESTABLISHES NEW OIL PROVINCE IN THE EASTERN TIMOR SEA (TIMOR GAP ZONE OF COOPERATION)

1995 ◽  
Vol 35 (1) ◽  
pp. 44
Author(s):  
I. F. Young ◽  
T.M. Schmedje ◽  
W.F. Muir
Keyword(s):  
Seismic Data ◽  
Late Jurassic ◽  
Joint Venture ◽  
Seismic Survey ◽  
Normal Faults ◽  
Commercial Development ◽  
The North ◽  
3D Data ◽  
Porosity And Permeability ◽  
Timor Sea

The Elang-1 oil discovery in the Timor Gap Zone of Cooperation (ZOC) has established a new oil province in the eastern Timor Sea. The discovery well, completed in February 1994, recorded a flow of 5,800 BOPD (5,013 STBOPD) from marine sandstone of the Late Jurassic Montara beds. The oil is a light (56° API), undersaturated oil with a GOR of approximately 550 SCF/STB. Elang-1 was the first well drilled by the ZOCA 91-12 Joint Venture and only the fifth well in the ZOC since exploration of this frontier area resumed in 1992.The Elang Prospect, initially mapped by Petroz in the late 1970s on the basis of regional seismic data, was detailed by the 1992 Walet Seismic Survey. The prospect is the main crestal culmination on the Elang Trend, a prominent structural high to the north of the Flamingo High that was established during continental break-up in the Late Jurassic. The Elang Trend is bounded to the south by a series of en-echelon normal faults and connecting relay ramps and comprises a number of horst and tilted fault blocks.Elang-1 tested a near crestal culmination on the Elang Prospect and intersected a 76.5 m gross oil column below 3,006.5 m RT. At time of drilling this oil column was the thickest that had been encountered by any well in the Northern Bonaparte Basin. Good quality reservoir sandstone in six discrete bodies were intersected within the Montara beds. Core-measured porosity and permeability range up to 17 per cent and 2.2 Darcies within the oil column.Subsequent to the Elang discovery, the Joint Venture recorded a 402 km2 3D survey over the Elang Trend. Elang-2, an appraisal well spudded in September 1994 prior to receipt of the 3D data, established the lateral continuity of the Montara beds reservoirs. Flow rates of 6,080 BOPD (5,300 STBOPD) and 7,500 BOPD (5,970 STBOPD) from separate intervals have confirmed that high deliverabilities can be expected from individual sandstones. Further appraisal drilling is planned in the first half of 1995. This is expected to lead to commercial development of the field.

scholarly journals Superposition of two kinematically distinct extensional phases in southern Death Valley: Implications for extensional tectonics

Geosphere ◽  
2021 ◽  
Author(s):  
Z.D. Fleming ◽  
T.L. Pavlis ◽  
S. Canalda
Keyword(s):  
Early Period ◽  
Rock Avalanche ◽  
Strike Slip ◽  
Death Valley ◽  
Normal Faults ◽  
Geologic Mapping ◽  
The North ◽  
Transcurrent Deformation ◽  
Two Phases

Geologic mapping in southern Death Valley, California, demonstrates Mesozoic contractional structures overprinted by two phases of Neogene extension and contemporaneous strike-slip deformation. The Mesozoic folding is most evident in the middle unit of the Noonday Formation, and these folds are cut by a complex array of Neogene faults. The oldest identified Neogene faults primarily displace Neoproterozoic units as young as the Johnnie Formation. However, in the northernmost portion of the map area, they displace rocks as young as the Stirling Quartzite. Such faults are seen in the northern Ibex Hills and con­sist of currently low- to moderate-angle, E-NE– dipping normal faults, which are folded about a SW-NE–trending axis. We interpret these low-angle faults as the product of an early, NE-SW extension related to kinematically similar deformation recognized to the south of the study area. The folding of the faults postdates at least some of the extension, indicating a component of syn-exten­sional shortening that is probably strike-slip related. Approximately EW-striking sinistral faults are mapped in the northern Saddlepeak Hills. However, these faults are kinematically incompatible with the folding of the low-angle faults, suggesting that folding is related to the younger, NW-SE extension seen in the Death Valley region. Other faults in the map area include NW- and NE-striking, high-angle normal faults that crosscut the currently low-angle faults. Also, a major N-S–striking, oblique-slip fault bounds the eastern flank of the Ibex Hills with slickenlines showing rakes of <30°, which together with the map pattern, suggests dextral-oblique movement along the east front of the range. The exact timing of the normal faulting in the map area is hampered by the lack of geochronology in the region. However, based on the map relationships, we find that the older extensional phase predates an angular unconformity between a volcanic and/or sedimentary succession assumed to be 12–14 Ma based on correlations to dated rocks in the Owlshead Mountains and overlying rock-avalanche deposits with associated sedimentary rocks that we correlate to deposits in the Amargosa Chaos to the north, dated at 11–10 Ma. The mechanism behind the folding of the northern Ibex Hills, including the low- angle faults, is not entirely clear. However, transcurrent systems have been proposed to explain extension-parallel folding in many extensional terranes, and the geometry of the Ibex Hills is consistent with these models. Collectively, the field data support an old hypothesis by Troxel et al. (1992) that an early period of SW-NE extension is prominent in the southern Death Valley region. The younger NW-SE extension has been well documented just to the north in the Black Mountains, but the potential role of this earlier extension is unknown given the complexity of the younger deformation. In any case, the recognition of earlier SW-NE extension in the up-dip position of the Black Mountains detachment system indicates important questions remain on how that system should be reconstructed. Collectively, our observations provide insight into the stratigraphy of the Ibex Pass basin and its relationship to the extensional history of the region. It also highlights the role of transcurrent deformation in an area that has transitioned from extension to transtension.

Lower Cretaceous stratigraphic characteristics and tectonic control of the eastern depression, North Yellow Sea Basin, North China

2020 ◽  
Vol 57 (10) ◽  
pp. 1180-1192
Author(s):  
Zhen Zhang ◽  
Rihui Cheng ◽  
Yanjie Shen ◽  
Liaoliang Wang ◽  
Xiaoqiang Hu ◽  
...  
Keyword(s):  
Yellow Sea ◽  
Lower Cretaceous ◽  
North China ◽  
Strike Slip ◽  
Lateral Movement ◽  
Third Order ◽  
Reservoir Rocks ◽  
North Yellow Sea ◽  
The North

The Lower Cretaceous of the eastern depression in the North Yellow Sea Basin is a set of residual strata that can be divided into K1sq1 and K1sq2 sequences. There are four lithology–lithofacies architectures summarized in the third-order sequences of wells W5, W3, W1, W9, W16, W7, W8, and W10, and they are the coarse–fine–coarse, asymmetric coarse–fine, asymmetric fine–coarse, and interbedded coarse and fine. F1, F4, F6, and F7, which are strike-slip faults, were dominant during the Early Cretaceous, and controlled the eastern depression to undergo right-lateral movement from transtension to transpression. The tectonic movement controlled different stratigraphic structure in different areas, and the fan bodies deposited along the basin margin and progradated into the basin center. The sequence models under extensional and strike-slip setting were established respectively. The transtension–transpression movement controlled the development of the sandstones in the Lower Cretaceous and improved the quality of the reservoir rocks.

Strain partitioning around an indenter during oroclinal bending: kinematics of the Circum-Moesian fault system of the Carpatho-Balkanides

2021 ◽  
Author(s):  
Nemanja Krstekanic ◽  
Liviu Matenco ◽  
Uros Stojadinovic ◽  
Ernst Willingshofer ◽  
Marinko Toljić ◽  
...  
Keyword(s):  
Large Scale ◽  
Strike Slip ◽  
Fault System ◽  
Normal Faults ◽  
Strain Partitioning ◽  
The South ◽  
The Carpathians ◽  
The North ◽  
Moesian Platform ◽  
Parallel Extension

<p>The Carpatho-Balkanides of south-eastern Europe is a double 180° curved orogenic system. It is comprised of a foreland-convex orocline, situated in the north and east and a backarc-convex orocline situated in the south and west. The southern orocline of the Carpatho-Balkanides orogen formed during the Cretaceous closure of the Alpine Tethys Ocean and collision of the Dacia mega-unit with the Moesian Platform. Following the main orogen-building processes, the Carpathians subduction and Miocene slab retreat in the West and East Carpathians have driven the formation of the backarc-convex oroclinal bending in the south and west. The orocline formed during clockwise rotation of the Dacia mega-unit and coeval docking against the Moesian indenter. This oroclinal bending was associated with a Paleocene-Eocene orogen-parallel extension that exhumed the Danubian nappes of the South Carpathians and with a large late Oligocene – middle Miocene Circum-Moesian fault system that affected the orogenic system surrounding the Moesian Platform along its southern, western and northern margins. This fault system is composed of various segments that have different and contrasting types of kinematics, which often formed coevally, indicating a large degree of strain partitioning during oroclinal bending. It includes the curved Cerna and Timok faults that cumulate up to 100 km of dextral offset, the lower offset Sokobanja-Zvonce and Rtanj-Pirot dextral strike-slip faults, associated with orogen parallel extension that controls numerous intra-montane basins and thrusting of the western Balkans units over the Moesian Platform. We have performed a field structural study in order to understand the mechanisms of deformation transfer and strain partitioning around the Moesian indenter during oroclinal bending by focusing on kinematics and geometry of large-scale faults within the Circum-Moesian fault system.</p><p>Our structural analysis shows that the major strike-slip faults are composed of multi-strand geometries associated with significant strain partitioning within tens to hundreds of metres wide deformation zones. Kinematics of the Circum-Moesian fault system changes from transtensional in the north, where the formation of numerous basins is controlled by the Cerna or Timok faults, to strike-slip and transpression in the south, where transcurrent offsets are gradually transferred to thrusting in the Balkanides. The characteristic feature of the whole system is splaying of major faults to facilitate movements around the Moesian indenter. Splaying towards the east connects the Circum-Moesian fault system with deformation observed in the Getic Depression in front of the South Carpathians, while in the south-west the Sokobanja-Zvonce and Rtanj-Pirot faults splay off the Timok Fault. These two faults are connected by coeval E-W oriented normal faults that control several intra-montane basins and accommodate orogen-parallel extension. We infer that all these deformations are driven by the roll-back of the Carpathians slab that exerts a northward pull on the upper Dacia plate in the Serbian Carpathians. However, the variability in deformation styles is controlled by geometry of the Moesian indenter and the distance to Moesia, as the rotation and northward displacements increase gradually to the north and west.</p>

scholarly journals Fault evolution in the Potiguar rift termination, equatorial margin of Brazil

Solid Earth ◽  
2015 ◽  
Vol 6 (1) ◽  
pp. 185-196 ◽  
Author(s):  
D. L. de Castro ◽  
F. H. R. Bezerra
Keyword(s):  
Sedimentary Basins ◽  
Strike Slip ◽  
South Atlantic ◽  
Normal Faults ◽  
South American ◽  
Sedimentary Sequences ◽  
The North ◽  
Lateral Shearing ◽  
The Right ◽  
Fault Evolution

Abstract. The transform shearing between South American and African plates in the Cretaceous generated a series of sedimentary basins on both plate margins. In this study, we use gravity, aeromagnetic, and resistivity surveys to identify architecture of fault systems and to analyze the evolution of the eastern equatorial margin of Brazil. Our study area is the southern onshore termination of the Potiguar rift, which is an aborted NE-trending rift arm developed during the breakup of Pangea. The basin is located along the NNE margin of South America that faces the main transform zone that separates the North and the South Atlantic. The Potiguar rift is a Neocomian structure located at the intersection of the equatorial and western South Atlantic and is composed of a series of NE-trending horsts and grabens. This study reveals new grabens in the Potiguar rift and indicates that stretching in the southern rift termination created a WNW-trending, 10 km wide, and ~ 40 km long right-lateral strike-slip fault zone. This zone encompasses at least eight depocenters, which are bounded by a left-stepping, en echelon system of NW–SE- to NS-striking normal faults. These depocenters form grabens up to 1200 m deep with a rhomb-shaped geometry, which are filled with rift sedimentary units and capped by postrift sedimentary sequences. The evolution of the rift termination is consistent with the right-lateral shearing of the equatorial margin in the Cretaceous and occurs not only at the rift termination but also as isolated structures away from the main rift. This study indicates that the strike-slip shearing between two plates propagated to the interior of one of these plates, where faults with similar orientation, kinematics, geometry, and timing of the major transform are observed. These faults also influence rift geometry.

scholarly journals Aptian faulting in the Haushi-Huqf (Oman) and the tectonic evolution of the southeast Arabian platform-margin

GeoArabia ◽  
2003 ◽  
Vol 8 (4) ◽  
pp. 643-662 ◽  
Author(s):  
Christian Montenat ◽  
Pascal Barrier ◽  
Henri J. Soudet
Keyword(s):  
Strike Slip ◽  
Coarse Grained ◽  
Normal Faults ◽  
Third Order ◽  
Lower Aptian ◽  
Platform Margin ◽  
Platform Carbonates ◽  
Transform Margin ◽  
Order Sequence ◽  
Arabian Platform

ABSTRACT A major upper Aptian unconformity is recorded on the eastern Arabian Platform, between the lower Aptian Qishn limestone and the Albian Nahr Umr marls. The study of this hiatus, in the western homocline of the Haushi-Huqf Uplift (Eastern Central Oman) provides new data about the evolution of the eastern Arabian Platform during middle Cretaceous times. The limestones of the Qishn formed a shoaling sequence, mainly composed of matrix-rich, coarse-grained sediment with small rudistids and algal build-ups, that led to a subemergent environment. A third-order sequence is recognized in the Qishn platform carbonates, which is partitioned into three minor sequences. The Qishn carbonate was subjected to pre-lithification normal faulting. A thick ferrugineous crust (hardground) covered the top surface of the Qishn as well as the faultscarps before they were buried under the Albian Nahr Umr marls. The faults are dominantly NW-trending, SW-facing, normal faults. The significance of the faulting remains hypothetical. The syndiagenetic NW-SE normal faults may correspond to ‘en-echelon’ faults, combined with a sinistral movement of the Haushi-Nafun Fault (HNF). The HNF acted as a left-lateral, strike-slip fault during late Cretaceous, pre-Maastrichtian times. This movement possibly began earlier, during the late Aptian. It could be related to the dynamics of the eastern Arabian margin during the Cretaceous (Masirah transform margin). There are some indications testifying to the activity of the Masirah transform fault during the early-middle Cretaceous. The margin kinematics may be responsible for the reactivation of nearby large faults affecting the platform basement (for instance the HNF). A slight sinistral reactivation of the HNF may have induced the development of the Aptian NW-trending normal faults. Moreover, the occurrence of early Cretaceous strike-slip movements in the Arabian Platform have already been envisaged, at a plate-scale, as a consequence of the South Atlantic extension. On this assumption, the Aptian fault blocks may have resulted from the development of a sinistral transtension along the HNF.

New Viewpoint on the Geology and Hydrocarbon Prospectivity of the Seychelles Plateau

2016 ◽  
Author(s):  
James Granath ◽  
Rolf Rango ◽  
Pete Emmet ◽  
Colin Ford ◽  
Robert Lambert ◽  
...  
Keyword(s):  
North Sea ◽  
Low Frequency ◽  
Hard Water ◽  
South Coast ◽  
Normal Faults ◽  
The South ◽  
East African ◽  
The North ◽  
Hydrocarbon Traps ◽  
Conjugate Margins

ABSTRACT We have reprocessed, re-imaged, and interpreted 10000+ km of legacy 2D seismic data in the Seychelles, particularly in the western part of the Plateau. Seychelles data have been difficult to image, particularly for the Mesozoic section: volcanics are a major attenuator of low frequency signal, and a hard water bottom contributes to signal problems. Enhanced low frequency techniques were applied to improve the signal fidelity in the 4 to 20 Hz range, and to remove spectral notches of shallow geologic origin. These efforts have allowed a reasonable view of the structure of the Plateau to a depth equivalent to about 3.5 sec TWT, and permit a comparison of areas atop the Plateau to the south coast where the three 1980's Amoco wells were drilled. It is clear that the main Plateau area of the Seychelles (excluding the outlying territories) is comprised of several separate basins, each with similar Karoo, Cretaceous, and Cenozoic sections that relate to the East African and West Indian conjugate margins, but the basins each have nuanced tectono-stratigraphic histories. The previously recognized Correira Basin in the SE and the East and West South Coast Basins face the African conjugate margin; other unimaged ones complete the periphery of the Plateau. The interior of the Plateau is dominated by the Silhouette Basin to the west of the main islands and the Mahé Basin to the east. The co astal basins have harsh tectono-thermal histories comparable to other continental margins around the world; they are typically characterized by stretching, subsidence and breakaway from their respective conjugate margins. In contrast the interior basins are comparable to ‘failed’ rift systems such as the North Sea or the Gulf of Suez. The South Coastal Basins, for example, tend to be more extended which complicated interpretation of the Amoco wells, but they have significant upside, as exemplified by the Beau Vallon structure. The interior basins, on the other hand, have typically simpler structure: the Silhouette Basin contains a system of NW-trending linked normal faults that could easily harbor North Sea-sized hydrocarbon traps with a variety of rift-related reservoir possibilities. Bright, reflective, hard volcanic horizons are less common than usually presumed, but most of the basins may contain considerable pyroclastic material in parts of the section. All of the basins appear to be predominantly oil prone, with considerable upside prospectivity.

scholarly journals Structural complexity in the boundary of forearc basin – accretionary wedge in the northwesternmost Sunda active margin

2018 ◽  
Vol 33 (1) ◽  
Author(s):  
Maruf M. Mukti
Keyword(s):  
Structural Complexity ◽  
Strike Slip ◽  
Accretionary Wedge ◽  
Strain Partitioning ◽  
Forearc Basin ◽  
Eastern Margin ◽  
The North ◽  
Step Over ◽  
Oblique Convergent ◽  
North Sumatra

The area from Andaman to northern Sumatran margin is a region where major faults collided that complicates the structural configuration. The origin of structures in the boundary between the accretionary wedge and forearc basin in the northwesternmost segment of the Sunda margin has been a subject of debates. This article reviews several published works on the Andaman – north Sumatran margin to characterize the boundary between forearc basin and accretionary wedge. Complex strain partitioning in this margin is characterized by sliver faults that crossing boundaries between the backarc basin, volcanic arc, forearc basin, and accretionary wedge. The fault zone can be divided into two segments: The West Andaman Fault (WAF) in the north and Simeulue Fault (SiF) in the southern part. A restraining step-over formed in between WAF and SiF. The SiF may extent onshore Simeulue to a strike-slip fault onshore. Strain-partitioning in such an oblique convergent margin appears to have formed a new deformation zone rather than reactivated the major rheological boundary in between the accretionary wedge and forearc basin. The eastern margin of the Andaman-north Sumatra accretionary wedge appears to have form as landward-vergent backthrusts of Diligent Fault (DF) and Nicobar Aceh Fault (NAF) rather than strike-slip faults. This characteristic appears to have formed in the similar way with the compressional structures dominated the eastern margin accretionary wedge of the central and south Sumatra forearc. Keywords: Andaman, North Sumatra, forearc, structure, accretionary wedge, strain partitioningDaerah Andaman - Sumatera bagian utara adalah wilayah di mana patahan-patahan besar saling bertemu dan membuat konfigurasi struktur menjadi rumit. Asal-usul struktur di batas antara prisma akresi dan cekungan busur muka di bagian paling baratlaut dari tepian Sunda telah menjadi topik perdebatan. Artikel ini mengulas beberapa studi yang telah diterbitkan sebelumnya mengenai tepian Andaman - Sumatra bagian utara untuk mengkarakterisasikan batas antara cekungan muka dan prisma akresi. Pemisahan regangan yang kompleks di tepian ini dicirikan oleh sliver fault yang melintasi batas antara cekungan busur belakang, busur vulkanik, cekungan busur muka, dan prisma akresi. Zona sesar tersebut dapat dibagi menjadi dua segmen, yaitu Sesar Andaman Barat (WAF) di utara dan Simeulue Fault (SiF) di bagian selatan. Sebuah restraining step-over terbentuk di antara WAF dan SiF. SiF kemungkinan menerus sampai ke Pulau Simeulue dan menyatu dengan sesar geser. Pemisahan regangan di tepian konvergen yang miring seperti itu tampaknya telah membentuk zona deformasi baru daripada mengaktifkan kembali batas reologi utama di antara prisma akresi dan cekungan busur muka. Batas bagian timur dari prisma akresi di Andaman – Sumatera bagian utara memiliki bentuk sebagai backthrusts berarah darat yaitu Sesar Diligent (DF) dan Sesar Nicobar Aceh (NAF) dan bukan merupakan sesar geser. Karakteristik ini tampaknya terbentuk dengan proses yang mirip dengan struktur-struktur kompresional yang mendominasi bagian timur prisma akresi di daerah Sumatra bagian tengah dan selatan.Kata kunci: Andaman, Sumatera bagian, busur muka, struktur, prisma akresi, pemisahan regangan 

Time-transgressive fault evolution and its impact on trap integrity: Timor Sea examples

2010 ◽  
Vol 50 (2) ◽  
pp. 701
Author(s):  
Bozkurt Ciftci ◽  
Laurent Langhi
Keyword(s):  
Late Jurassic ◽  
Normal Faults ◽  
Seal Integrity ◽  
Seal Failure ◽  
Hydrocarbon Traps ◽  
Exploration Risk ◽  
Timor Sea ◽  
Downward Propagation ◽  
Fault Growth ◽  
Fault Evolution

Top and fault seal failure represents an exploration risk in the Timor Sea where hydrocarbons are typically associated with hourglass structures. These structures comprise two distinct systems of conjugate normal faults that formed by 1st-phase (late Jurassic) and 2nd-phase (Neogene) extensions. Horst blocks bounded by 1st-phase faults potentially trap hydrocarbons and are overlain by grabens bounded by 2nd-phase faults. The two fault systems generally merge and intersect in dip direction to form the composite and time-transgressive faults of the hourglass structures. The 2nd-phase of extension is seen as the dominant cause of the seal breach. Revaluation of a series of hourglass structures on the Laminaria High confirmed two distinct sections of syn-kinematic strata. Bases of these sections correspond to maximum throws on the fault planes where the faults were probably nucleated. The presence of negative throw gradients upward and downward from the throw maximums indicate syn-kinematic deposition and fault growth, respectively. Assessment of these trends suggests that the 1st and 2nd-phase faults were detached at the onset of the 2nd-phase of extension. Connection was predominantly established by down-dip growth of the 2nd-phase faults while the reactivation of the 1st-phase faults may have remained minor. Seismic evidence of leakage from attribute mapping is used to constrain the timing of fault linkage and to validate prediction of leaking fault planes. It was noted that downward propagation of the 2nd-phase faults towards the hydrocarbon traps stresses the top seal integrity due to fault tip deformation front and development of sub-seismic fractures.

THE STRESS FIELD OF THE NORTH WEST SHELF AND WELLBORE STABILITY

1993 ◽  
Vol 33 (1) ◽  
pp. 373 ◽  
Author(s):  
R.R. Millis ◽  
A.F. Williams
Keyword(s):  
Stress Field ◽  
Horizontal Stress ◽  
Strike Slip ◽  
Wellbore Stability ◽  
Upper And Lower Bounds ◽  
Theoretical Modelling ◽  
Elliptical Cross ◽  
North West ◽  
The North ◽  
Timor Sea

Boreholes drilled in the search for hydrocarbons in the Barrow-Dampier Sub-Basin (North West Shelf, Australia) commonly exhibit an elliptical cross-section believed to be due to stress-induced wellbore failure known as borehole breakout. The azimuths of the long axes of 138 discrete breakouts identified in nine different wells in the Barrow-Dampier show a consistent 010°−030°N trend implying that maximum horizontal compressive stress is oriented 100°−12G°N.The orientation of horizontal stress determined in this study (and that from the Timor Sea area which is rotated some 50°−60° with respect to the Barrow-Dampier) is consistent with that derived from theoretical modelling of the stress within the Indo-Australian plate based on the plate tectonic forces acting on its boundaries. The rotation of the horizontal stress orientations along the North West Shelf, between the Barrow-Dampier and the Timor Sea, is a reflection of the present-day complex plate convergence system at the north-eastern boundary of the Indo-Australian Plate.Vertical stress magnitudes, Sv, in the Barrow-Dampier were determined from density and sonic log data. Minimum and maximum horizontal stress magnitudes, Shmin and Shmax, were determined from mini-hydraulic fracture (or modified leak-off) test results. These data suggest that the fault condition of the Wanaea/Cossack area is on the boundary between normal faulting (extension) and strike-slip, i.e. Sv ≈ Shmax > Shmin. However, in other parts of the Barrow-Dampier the evidence suggests a strike-slip fault condition, i.e. Shmax > Sv > Shmin.Given the orientation of the stress field and the fault condition, inferences can be drawn regarding the stability of horizontal wells. Furthermore, experience from vertical wells can be utilized to determine the upper and lower bounds to the mud-weight envelope as functions of deviation and wellbore orientation. Since a horizontal well will see Sv and a horizontal stress, stress anisotropy around a wellbore in the Wanaea/Cossack area (and hence wellbore instability) will be minimized by drilling in the Shmin direction i.e. 010°–030°N.

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