Cuplikan dari Paper ttg West Borneo dibawah, 2011/3/1 <[email protected]>
> Vita, > > > > Untuk Kutei saya tidak tau banyak, tapi untuk Brunei shelf banyak sekali > overpressure study dan banyak yang published. Yang banyak publish adalah > Mark Tingay. Contohnya: > > http://geology.geoscienceworld.org/cgi/content/abstract/35/11/1023 > > > > Mungkin bisa jadi analog. > > Salam, > *Marine and Petroleum Geology*<http://www.sciencedirect.com/science/journal/02648172> Volume 21, Issue 7<http://www.sciencedirect.com/science?_ob=PublicationURL&_tockey=%23TOC%235911%232004%23999789992%23514239%23FLA%23&_cdi=5911&_pubType=J&view=c&_auth=y&_acct=C000059517&_version=1&_urlVersion=0&_userid=5194839&md5=58264278367ad58fa553c552c4ba5aae>, August 2004, Pages 879-887 *Deepwater North West Borneo: hydrocarbon accumulation in an active fold and thrust belt * *G. M. Ingram[image: Corresponding Author Contact Information]<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#m4.cor*>, a<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#aff1>, T. J. Chisholma<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#aff1>, C. J. Granta<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#aff1>, C. A. Hedlundb<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#aff2>, P. Stuart-Smithc<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#aff3>and J. Teasdale c<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#aff3> * Langsung diskusinya :) RDP ====================================== 6. Discussion and case examples The contractional deformation in North West Borneo has exerted fundamental influences upon three important elements of the hydrocarbon system in the basin: • Trap integrity• Hydrocarbon migration• Turbidite sand deposition In terms of trap integrity, structures in the Toe Thrust play are exposed to a high risk of crestal leakage. In the worst case this can result in the loss of a substantial part of the hydrocarbon fill of a trap via a hydrofracturing mechanism in which leakage occurs through extensional fracture networks. Such permeable extensional fracture networks are likely to form in response to elevated pore fluid pressures in excess of the in situ minimum stress. Relative increase of pore fluid pressure can occur due to a number of processes including inflation, disequilibrium compaction, buoyancy of a hydrocarbon column, tectonic uplift and erosion of sediment overburden. 6.1. Case examples The result of a Shell exploration well in Sabah Block G has led to the conclusion that the trap was breached, or ‘blown,’ through the combined effects of mild background overpressure (inflation), tectonic uplift, erosion of overburden and a buoyancy of a large gas column. The well encountered a substantial residual gas column in the Kinarut objective formation that is thought to have resulted from leakage of the originally trapped hydrocarbons subsequent to a rapid phase of compressional deformation and related uplift. Seismic interpretation indicated that the tectonic uplift was combined with rapid erosion at the sea floor and as a result pressures at the crest of the trap were brought into mechanical disequilibrium with the sediment overburden. This is likely to have resulted in catastrophic leakage via mode I fracture networks while the state of mechanical disequilibrium was maintained at the crest of the structure. Hydrocarbon discoveries elsewhere in North West Borneo, in contrast, appear to have retained their hydrocarbon fill because: • mechanical pressure equilibrium was maintained by virtue of a thick overburden,• the trap was re-filled via later hydrocarbon charge, or• the trap is not located in an active high strain zone. The Kamunsu East Upthrown (KMEU) gas field is situated in the NE corner of Block G (Fig. 1<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#fig1>) in water depths of 1000–1130 m and was discovered in 1999 by well KMEU-1st1, which found gas and oil in Upper Miocene turbidite reservoirs. This field provides an example of a trap that has leaked, or ‘blown’, and resealed again. The trap is an elongate thrust fault propagation fold structure dislocated along its western and northern flanks by a thrust fault showing evidence of recent reactivation. Hydrocarbons are trapped in multiple turbidite reservoirs of mid-late Miocene age. The structure was first targeted by exploration well KMEU-1st1/2, which made a small, non-commercial gas discovery in the Pink Fan complex. The main objective of this first test was a sub-thrust footwall dip closure at the Kinarut fan level. The well targeted a prominent seismic flat spot but found only residual hydrocarbons (oil and gas), suggesting trap failure due to suspected top-seal breaching. KMEU Canyon-1 (KMEUC-1) and geological sidetrack were drilled in mid-2002 and targeted the crestal part of the structure. The main appraisal target was the Pink fan Canyon-fill complex. An exploratory sidetrack was drilled to search for further hydrocarbons in the Kinarut attic prospect in an area obscured on seismic data by the distorting effects of shallow gas leaking from the crest of the structure. The exploratory sidetrack found full saturation gas in the crest of the structure at the Kinarut level. Pressure analysis reveals that the current hydrocarbon column length, estimated to be in the order of 300 m including a possible 90 m oil rim, is the maximum the structure can hold assuming low top seal strength. These well results have shown that leaky toe-thrust traps can leak and reseal thereby retaining long hydrocarbon columns. 6.2. Tectonic influences on hydrocarbon migration Tectonic activity also has an important influence on hydrocarbon migration. In the early stages of migration hydrocarbons will enter early-formed toe thrusts and folds. These traps will tend to focus migration and, with continued deformation, they may become important ‘apex’ structures, which typically have very high structural relief and large fetch area combined with very limited overburden and hence compromised trap integrity. Early-formed traps not modified by subsequent deformation phases will remain buried and will tend to spill hydrocarbons via their structural spill points. In these cases oil may be lost preferentially. Differential leakage of the kinds discussed here may exert control on the onward migration of the separate oil and gas phases from a given fetch area. Hydrocarbon fluids in any given part of the basin will tend to migrate towards the major apex structures representing the ultimate leak points for fluids to the free surface. These leak points will also serve to regulate background aquifer pressure in cases where reservoirs are connected and laterally extensive. In this way it may be argued that continued deformation in North West Borneo has a fundamental influence on hydrocarbon migration and trap fill. In addition to leakage and migration through top seals, faults are often proposed as fundamental pathways for hydrocarbon migration although specific mechanisms for fluid transport are often unclear. More specifically, faults, or fault zones, may act as conduits for hydrocarbon migration, given the ideal physical conditions. This is supported by the results of seabed geochemical sampling surveys carried out in Borneo over recent years, which show that oil and gas seepage commonly occurs above shallow faults. In order to transmit fluids, an inactive fault zone must be more permeable than the surrounding wall rock. In the case of active faults, dilatant behaviour caused by fault slip reactivation can create along-fault permeability and permit along-fault fluid flow. Such dilatant strain behaviour is much more likely to take place in strong, brittle rocks than weak, ductile rocks (Ingram & Urai, 1999<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#bib6>). However, the rheology of the young Neogene rocks in North West Borneo is relatively weak and therefore it is not so clear that these ductile sediments would indeed dilate under conditions of fault reactivation. It has been observed that strain related to permeability enhancement in fault zones may manifest itself in the form of closely-spaced fractures in clay-rich fault gouge material, as seen in cores from the Norwegian Continental Shelf ( Harper & Lundin, 1997<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#bib4>). Dilatant behaviour has also been linked to the breaching of Jurassic-age traps in the Timor Sea area of Australia, controlled by Neogene age fault reactivation resulting from the collision of Australia with SE Asia ( Shuster, Eaton, Wakefield, & Kloosterman, 1998<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#bib13>). In addition to actively slipping faults, critically stressed faults are also considered to be prone to act as conduits for fluid flow ( Jones & Hillis, 2003<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#bib7>). It may be argued that critically stressed faults have enhanced permeability caused by the accumulation of precursor strain prior to slippage. At any one time, the Tertiary sediment pile in North West Borneo is likely to contain both active and critically stressed faults, therefore it may be argued that permeable fault zones may exist and hydrocarbons may migrate along them. If faults are indeed the major control on migration (including trap filling and leakage) in North West Borneo, then a mechanism other than brittle dilation may be responsible. 6.3. Tectonic influences on fan deposition Active tectonics has also exerted fundamental control on the deposition of the major turbidite fan units in the deep water of the North West Borneo Basin. Work by Casson<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#bbib1>, N., Wannier, M., Lobao, J., & George, P. (1999). *Modern morphology—ancient analogue: insights into deep-water sedimentation on the active tectonic margin of West Sabah*. GEOSEA '98 Proceedings, Geological Society Malaysia Bulletin 43. pp. 399–403.Casson, Wannier, Lobao, and George (1999)<http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-4CDHH47-2&_user=3034635&_coverDate=08%2F31%2F2004&_rdoc=1&_fmt=high&_orig=gateway&_origin=gateway&_sort=d&_docanchor=&view=c&_searchStrId=1659981175&_rerunOrigin=google&_acct=C000059517&_version=1&_urlVersion=0&_userid=3034635&md5=bf7488e2bff2f672a9581eb7e3191c3c&searchtype=a#bib1>showed that tectonic activity significantly enhanced the effects of eustatic sea level falls, leading to intense reworking of shelf sediments into deep water and as a result, numerous, high energy, sand-rich base-of-slope fans developed. This intense fan development is contrasted by periods of relatively low tectonic activity in which sea level falls fail to expose the entire shelf and sediments are deposited in relatively narrow shelf areas as low energy mud-rich fan systems. 7. Conclusions Oil and gas discoveries in the deepwater North West Borneo Basin show that hydrocarbon accumulation has occurred in spite of the active deformation of the deepwater fold and thrust belt. The basin still remains relatively under-explored, but is currently the focus of aggressive exploration campaigns. The most successful trapping geometries observed in the deepwater part of this Tertiary fold and thrust belt are buried hanging-wall anticlines formed by thrust-related fault propagation folds. Key challenges for operators include trap integrity and reservoir sand prediction, both of which are fundamentally related to the combination of deformation history and eustatic sea level fluctuations. Greater understanding of the complex migration-fill-spill history in the deep water of North West Borneo has been made possible from the analysis of partially breached hydrocarbon fields such as Kamunsu East Upthrown (KMEU). In the case of KMEU there exists a geophysical direct hydrocarbon indicator, which corresponds to a palaeo-contact left behind after a phase of top seal breaching related to tectonic uplift and erosion. Only after a well was drilled into the crest of the structure, through a significant gas chimney caused by the trap breaching event, did the existence of full saturation hydrocarbons above the palaeo-contact become known. In this paper it has been shown that significant knowledge has been gained from the few existing deepwater exploration wells in North West Borneo and that substantial volumes of hydrocarbons are likely to be discovered in the future. rdp -- *"Success is a mind set, not just an achievement"*
