DISCOVERY OF SALT IN THE VULCAN GRABEN: A GEOPHYSICAL AND GEOLOGICAL EVALUATION

1991 ◽  
Vol 31 (1) ◽  
pp. 229 ◽  
Author(s):  
P.M. Smith ◽  
N.D. Sutherland
Keyword(s):  
Late Jurassic ◽  
Structural Model ◽  
Magnetic Data ◽  
Salt Diapir ◽  
Tectonic Event ◽  
North West ◽  
Cap Rock ◽  
Collapse Structures ◽  
Structural Closure ◽  
Gravity And Magnetic Data

The Paqualin-1 well in Permit AC/P2, Timor Sea, was drilled to test a large structural closure against the flank of an interpreted piercement structure located in the Late Jurassic Paqualin Graben. Prior to drilling the well interpretation of geological, seismic, gravity and magnetic data supported both a salt diapiric and/or an igneous intrusive structural model for the origin of the piercement feature. On drilling the Paqualin-1 well in December 1988, a 627 m thick evaporitic sequence was encountered in the post-rift Tertiary sequence indicating that the well had penetrated a salt overhang close to the main diapiric stock.The age of the evaporitic sequence is unclear but is considered coeval with Palaeozoic salt diapirs in the Bonaparte Basin to the east. Growth of the Paqualin diapir and a similar feature to the south, the Swan structure, which is also interpreted to be a salt diapir, appears to have been triggered initially by the Late Jurassic-Early Cretaceous breakup of the Australian north-west continental margin (doming and pillowing), and then again by a second major tectonic event in the Late Miocene associated with the collision between the Australian and Eurasian plates (diapir- ism and collapse structures).A distinctive cap rock occurs at the top of the evaporitic sequence characterised by an unusual accessory suite of primary and secondary minerals including euhedral magnetite, bipyramidal quartz, biotite, chlorite, sphene, amphibole and feldspar. The high magnetite component is considered responsible for the positive magnetic anomaly observed prior to drilling. The presence of magnetite and the other minerals in the cap rock appears to be related to the presence of exotic igneous material incorporated in the salt stock and the restricted activity of sulphate reducing bacteria in the crest of the diapir.The discovery of the Paqualin salt diapir and the interpretation of a similar structure in the adjacent Swan Graben has lead to the recognition of new play types related to the diapirs.

The prospectivity of the Cape Vogel Basin, Papua New Guinea

2019 ◽  
Vol 59 (2) ◽  
pp. 840
Author(s):  
Said Amiribesheli ◽  
Andrew Weller
Keyword(s):  
Petroleum System ◽  
Source Rocks ◽  
Magnetic Data ◽  
North West ◽  
Late Mesozoic ◽  
Gulf Of Papua ◽  
Differential Uplift ◽  
Uplift And Erosion ◽  
Cenozoic Sediments ◽  
Gravity And Magnetic Data

The frontier and underexplored Cape Vogel Basin (CVB), north of the Papuan Peninsula, is thought to be underlain by Late Palaeocene–Eocene oceanic crust and overlain by Cenozoic sediments. Several impartial data provide evidence of working petroleum system(s) including a flow of oil from a 1920s well, and two 1970s wells that encountered minor hydrocarbon traces and good source material. The 1970s wells chased Miocene reef plays (like the discoveries in the Gulf of Papua). No Miocene reefs were encountered, with both wells terminating in volcanics. Integration of open-file 2D seismic, modern 2D PSDM seismic and shipborne gravity and magnetic data improves the subsurface imaging and thus understanding of prospectivity. The data reveal a significant sedimentary section (including Mesozoic sediments) and that the volcanics are not laterally continuous (i.e. products of short periods of volcanism). The data also suggests several Mesozoic–Cenozoic plays (e.g. carbonate reefs, incised canyons). Repeatable sea surface slicks, and observable bottom-simulating reflectors and direct hydrocarbon indicators, also provide evidence of working petroleum system(s). It is hypothesised that the CVB has affinities with the Gulf of Papua with the extension of the Australian craton north of the Papuan Peninsula, with widespread deposition in the Mesozoic–Cenozoic, and with source rocks estimated to be within the hydrocarbon generative window. With incorporation of onshore data and presence of significant gravity low, it is postulated that the central and north-west were less susceptible to Late Cretaceous and Palaeocene differential uplift and erosion (related to Coral Sea breakup and extension), and thus have a higher chance of Late Mesozoic preservation.

ACLAS — A method to define geologically significant lineaments from potential-field data

Geophysics ◽  
2017 ◽  
Vol 82 (4) ◽  
pp. G87-G100 ◽  
Author(s):  
Lorenzo Cascone ◽  
Chris Green ◽  
Simon Campbell ◽  
Ahmed Salem ◽  
Derek Fairhead
Keyword(s):  
Large Scale ◽  
Magnetic Data ◽  
Data Set ◽  
New Approach ◽  
Potential Field Data ◽  
North West ◽  
Gravity And Magnetic ◽  
Lineament Analysis ◽  
The Individual ◽  
Gravity And Magnetic Data

Geologic features, such as faults, dikes, and contacts appear as lineaments in gravity and magnetic data. The automated coherent lineament analysis and selection (ACLAS) method is a new approach to automatically compare and combine sets of lineaments or edges derived from two or more existing enhancement techniques applied to the same gravity or magnetic data set. ACLAS can be applied to the results of any edge-detection algorithms and overcomes discrepancies between techniques to generate a coherent set of detected lineaments, which can be more reliably incorporated into geologic interpretation. We have determined that the method increases spatial accuracy, removes artifacts not related to real edges, increases stability, and is quick to implement and execute. The direction of lower density or susceptibility can also be automatically determined, representing, for example, the downthrown side of a fault. We have evaluated ACLAS on magnetic anomalies calculated from a simple slab model and from a synthetic continental margin model with noise added to the result. The approach helps us to identify and discount artifacts of the different techniques, although the success of the combination is limited by the appropriateness of the individual techniques and their inherent assumptions. ACLAS has been applied separately to gravity and magnetic data from the Australian North West Shelf; displaying results from the two data sets together helps in the appreciation of similarities and differences between gravity and magnetic results and indicates the application of the new approach to large-scale structural mapping. Future developments could include refinement of depth estimates for ACLAS lineaments.

Automatic 3-D interpretation of potential field data using analytic signal derivatives

Geophysics ◽  
1997 ◽  
Vol 62 (1) ◽  
pp. 87-96 ◽  
Author(s):  
Nicole Debeglia ◽  
Jacques Corpel
Keyword(s):  
Signal Amplitude ◽  
Vertical Gradient ◽  
Analytic Signal ◽  
Magnetic Data ◽  
Practical Implementation ◽  
Potential Field Data ◽  
Gravity And Magnetic ◽  
Second Derivatives ◽  
Gravity And Magnetic Data ◽  
Derivatives Of

A new method has been developed for the automatic and general interpretation of gravity and magnetic data. This technique, based on the analysis of 3-D analytic signal derivatives, involves as few assumptions as possible on the magnetization or density properties and on the geometry of the structures. It is therefore particularly well suited to preliminary interpretation and model initialization. Processing the derivatives of the analytic signal amplitude, instead of the original analytic signal amplitude, gives a more efficient separation of anomalies caused by close structures. Moreover, gravity and magnetic data can be taken into account by the same procedure merely through using the gravity vertical gradient. The main advantage of derivatives, however, is that any source geometry can be considered as the sum of only two types of model: contact and thin‐dike models. In a first step, depths are estimated using a double interpretation of the analytic signal amplitude function for these two basic models. Second, the most suitable solution is defined at each estimation location through analysis of the vertical and horizontal gradients. Practical implementation of the method involves accurate frequency‐domain algorithms for computing derivatives with an automatic control of noise effects by appropriate filtering and upward continuation operations. Tests on theoretical magnetic fields give good depth evaluations for derivative orders ranging from 0 to 3. For actual magnetic data with borehole controls, the first and second derivatives seem to provide the most satisfactory depth estimations.

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