Cretaceous and Cenozoic Basin Evolution Based on Decompacted Sediment Thickness from Petroleum Wells around New Zealand

<p>Decompacted sedimentary data from 33 New Zealand exploration wells is used to investigate basin evolution and tectonics from around New Zealand. This analysis is directed to both a comparison of basin behaviour and a search for common subsidence signatures. Common to almost all New Zealand basin subsidence curves is a sedimentary signature associated with rifting of the Gondwana super-continent (80-65 Ma). In the Great South Basin a second rifting event is inferred at 51 [plus or minus] 2 Ma, illustrated by a rapid increase in subsidence rates (with a maximum rate of 190 m.Myr-1 at Pakaha-1). Coinciding with the cessation of Tasman Sea rifting ([approximately] 53 Ma), and with the onset of rifting in the Emerald Basin ([approximately] 50 Ma), it is assumed that the event is related to the tectonic plate reorganization. An increase in sedimentation is noted at [approximately] 20 Ma in most South Island wells. Convergence on the Alpine Fault, leading to increased erosion is cited as a mechanism for this period of basin growth, consistent with the Cande and Stock (2004) model of plate motions. A second increase in sedimentation occurs at [approximately] 6 Ma in almost all wells around New Zealand. Climate-driven erosion resulting in isostatic uplift is thought to contribute to this event. Hiatuses in the sedimentary record for the Canterbury, Great South and Western Southland Basins during the late Oligocene are interpreted as the Marshall Paraconformity. It appears that the break in sedimentation located within a regional transgressional mega-sequence was caused by mid Oligocene glacio-eustatic fall and related oceanic current processes. Loading by the Northland Allochthon, in conjunction with paleobathymetry and subsidence data, is used to demonstrate the mechanical properties of the lithosphere. A lithospheric rigidity of 1.5 x [10 to the power of 22] Nm is estimated, with an elastic thickness of 12 km. Considerably lower than elastic thickness values previously calculated for the Plio-Pleistocene loading of the Taranaki Platform. It is noted that the Northland value represents a younger, hotter crust at the time of load emplacment. With the exception of the central Taranaki and Great South Basins, stretching factors ([Beta]) for the sedimentary basins surrounding New Zealand are below 2. This suggests crustal thickness prior to rifting was between 35 and 50 km, consistent with data from conjugate margins of Australia and Antarctica. An increase in water depth in the Taranaki Basin at 25 [plus or minus] 3 Ma is confirmed by this study. This coincides with a similar signature on the West Coast of the South Island at 26 [plus or minus] 2 Ma. It is suggested that a mantle flow caused by the initiation of the subduction zone at [approximately] 25 Ma extends over a broader region (>750 km) than previously thought.</p>

Cretaceous and Cenozoic Basin Evolution Based on Decompacted Sediment Thickness from Petroleum Wells around New Zealand

<p>Decompacted sedimentary data from 33 New Zealand exploration wells is used to investigate basin evolution and tectonics from around New Zealand. This analysis is directed to both a comparison of basin behaviour and a search for common subsidence signatures. Common to almost all New Zealand basin subsidence curves is a sedimentary signature associated with rifting of the Gondwana super-continent (80-65 Ma). In the Great South Basin a second rifting event is inferred at 51 [plus or minus] 2 Ma, illustrated by a rapid increase in subsidence rates (with a maximum rate of 190 m.Myr-1 at Pakaha-1). Coinciding with the cessation of Tasman Sea rifting ([approximately] 53 Ma), and with the onset of rifting in the Emerald Basin ([approximately] 50 Ma), it is assumed that the event is related to the tectonic plate reorganization. An increase in sedimentation is noted at [approximately] 20 Ma in most South Island wells. Convergence on the Alpine Fault, leading to increased erosion is cited as a mechanism for this period of basin growth, consistent with the Cande and Stock (2004) model of plate motions. A second increase in sedimentation occurs at [approximately] 6 Ma in almost all wells around New Zealand. Climate-driven erosion resulting in isostatic uplift is thought to contribute to this event. Hiatuses in the sedimentary record for the Canterbury, Great South and Western Southland Basins during the late Oligocene are interpreted as the Marshall Paraconformity. It appears that the break in sedimentation located within a regional transgressional mega-sequence was caused by mid Oligocene glacio-eustatic fall and related oceanic current processes. Loading by the Northland Allochthon, in conjunction with paleobathymetry and subsidence data, is used to demonstrate the mechanical properties of the lithosphere. A lithospheric rigidity of 1.5 x [10 to the power of 22] Nm is estimated, with an elastic thickness of 12 km. Considerably lower than elastic thickness values previously calculated for the Plio-Pleistocene loading of the Taranaki Platform. It is noted that the Northland value represents a younger, hotter crust at the time of load emplacment. With the exception of the central Taranaki and Great South Basins, stretching factors ([Beta]) for the sedimentary basins surrounding New Zealand are below 2. This suggests crustal thickness prior to rifting was between 35 and 50 km, consistent with data from conjugate margins of Australia and Antarctica. An increase in water depth in the Taranaki Basin at 25 [plus or minus] 3 Ma is confirmed by this study. This coincides with a similar signature on the West Coast of the South Island at 26 [plus or minus] 2 Ma. It is suggested that a mantle flow caused by the initiation of the subduction zone at [approximately] 25 Ma extends over a broader region (>750 km) than previously thought.</p>

Are There Potentially Significant Long-Term Health Consequences of Exposure to Fine Airborne Particulate Matter PM10 to Personnel on the United Kingdom's Offshore Drilling Rigs?

Drilling mud is a slurry comprising oil, water, and chemical additives. Mud is critical to drilling a modern well as it is circulated down a wellbore to remove rock cuttings and to power the drill bit. A significant volume of this drilling mud is used and later recirculated. The drilling mud warms at depth, creating steam, which holds suspended PM and dissolved chemicals. Many of the pieces of equipment are open or only partially enclosed, allowing for steam generation, while other processes generate aerosolised sprays. There is a significant potential for petroleum workers to become exposed and potentially suffer health effects because of drilling mud exposure. This study aims to find the major sources of PM10 on petroleum wells and quantify the levels of exposure and health hazard associated with drilling mud on petroleum rigs. A literature search was performed, which included all available materials which contained static or mobile concentrations of PM10 or oil mist within the UK or international petroleum drilling sites with a preference for North Sea operations. The study predicts the total PM10 by estimating the combined impact of both solid PM and oil mist. Using this conversion, it is also possible to estimate PM10 concentrations when using water-based muds. The work designates and discusses the expected health ramifications of excess exposure. A quantitative assessment of the risk of silicosis 15 years post-exposure is also calculated, predicting dire consequences to petroleum personnel in the long term. The exposure assessment methods, hygienic standards, and preventive measures are also addressed briefly.

Interaction of extensional, contractional, and strike-slip elements at Mount Diablo and the surrounding eastern Coast Ranges, San Francisco Bay area, California: A model-based analysis

ABSTRACT This study presents three regional cross sections, a structural map analysis, and a schematic map restoration. The sections are constrained by surface geology and petroleum wells and were developed using model-based methods to be consistent with the regional tectonic context and balancing concepts. Together, these products depict the geometry and kinematics of the major fault systems. Insights from this research include the following. Franciscan complex blueschist-facies rocks in the Mount Diablo region were unroofed west of their current location and subsequently thrust beneath the Great Valley sequence in the mid-Eocene. East Bay structures are complicated by overprinting of Neogene compression and dextral strike-slip motion on a Paleogene graben system. Net lateral displacement between the Hayward fault and the Central Valley varies from 26 km toward 341° to 29 km toward 010° in the southern and northern East Bay Hills, respectively. Uplift above a wedge thrust generates the principal Neogene structural high, which extends from Vallejo through Mount Diablo to the Altamont Ridge. Anomalous structural relief at Mount Diablo is due to strike-parallel thrusting on the crest of a fault-propagation fold formed on the west-verging roof thrust. Uplift that exposes the Coast Range ophiolite in the East Bay Hills is formed by oblique thrusting generated by slip transfer at the northern termination of the Calaveras fault. The Paleogene extensional fault system likely extends farther west than previously documented. An east-dipping branch of that system may underlie the Walnut Creek Valley. Three-dimensional restoration should be applied to constrain geologic frameworks to be used for seismic velocity modeling.

Drilling optimization of petroleum wells

The petroleum industry is already demanding lowering exploration costs, which reflects in needs of reducing drilling operational costs, which can be achieved through implementation of more efficiency in operations. Researches have shown that scientist and companies are already experiencing different approaches aiming at boosting drillability. One method not well implemented is variation of flow-rate as a mechanical specific drilling parameter, given its complexity and relation to well integrity. This paper details the influence in flow-rate and related parameters in rate of penetration, showing how DHAP, ECD, Flow-rate and ROP are related to each other. It can be seen that by increasing a flow-rate, an increase in ROP is possible, but that flow-rate changes also influence the down-hole pressure and ECD, what imply in possible downhole differential pressure variation. The higher the down-hole differential pressure, the less will be the implied ROP. All these have influence on drilling efficiency.

Effect of Bentonite Prehydration Time on the Stability of Lightweight Oil-Well Cement System

Lightweight cement systems are used in the weak intervals of petroleum wells. Sodium bentonite is used as an extender in lightweight oil-well cement systems as it prevents excess water and sedimentation of particles, thereby ensuring the formation of homogenous and stable cement sheaths. The extending ability of sodium bentonite is enhanced when prehydrated. However, the optimum bentonite prehydration time and its effect on the stability of lightweight cement systems have not been well established. The objective of this study is to investigate the optimum sodium bentonite prehydration time and correlate it to the stability of lightweight oil-well cement systems. Bentonite suspensions were prepared by vigorous preshearing at 12000 rpm for 5 minutes, followed by aging times of 0, 30, 60, and 120 minutes. The swelling behavior of bentonite was investigated using a laser particle size analyzer. The Herschel-Bulkley model was used to determine the rheological parameters of the experimentally measured shear stress vs. shear rate data of the aged suspensions. The effect of calcium chloride salt on aged bentonite suspensions was investigated. Density measurements and pore space analysis with the nuclear magnetic resonance (NMR) technique were used to investigate the homogeneity of cement-based cores. It was observed that bentonite swells with time and, after 30 minutes, the swelling is insignificant; however, the swelling property did not have any observed impact on the properties of cement systems designed with the bentonite aged at different times. In general, all the lightweight cement slurries exhibited similar properties, in terms of rheology, stability, and homogeneity, regardless of the bentonite prehydration time. These findings indicate that aging bentonite suspension after vigorous preshearing in lightweight cement design is unnecessary and would only contribute to nonproductive time.