Regional Seismic Datasets Research Articles

Seismic imaging of Deccan-related lava flows at the K-T boundary, deepwater west India

Continuous improvements in geophysical technology have enabled seismic imaging of Mesozoic strata under the K-T transition boundary — a thick lava blanket spread along the continental margin of India's west coast. The new images reveal features of the subbasalt geology of this large offshore region. The relationship of the offshore lava flows with the equally vast lava flows of the onshore Deccan Traps adds a parameter of comparison to the whole Mesozoic stratigraphy of the south, west, and northwest coasts of India. The lava flows are part of the deepwater margin that embraces several basins, the largest being the southernmost underexplored Kerala-Konkan Basin. Newly acquired and processed regional seismic data sets were integrated with new data sets of potential field data to better uncover the Mesozoic stratigraphy. The 3D seismic data, acquired with long spreads and broadband processing through time and depth migration, enabled imaging of the complex structure of layers under basalt. India detached from Madagascar during Cretaceous time; the northward path to its present position was affected by Reunion hotspot activity that melted the west coast near the K-T transition time. The volcanic lithology observed at the K-T boundary is related to episodic emplacement of intrusive dykes and extrusive sills. A possible cause-effect event in that span of time is the Chicxulub meteorite impact that may have increased the volume of Deccan lava flows. The deepwater lava volumes estimated from interpretation of seismic data from the offshore region at the end of Cretaceous time are comparable to, or perhaps much greater than, the volumes estimated for the precisely age-dated Deccan Traps basalts exposed onshore. This suggests an enormous outpouring of flood basalts (onshore and offshore) at K-T time that was comparable to other major flood basalt events, such as the Siberian traps, and associated with mass extinctions.

Characterisation of focused gas hydrate accumulations from the Pegasus Basin, New Zealand, using high-resolution and conventional seismic data

Gas hydrates are reported widely in seismic data from New Zealand’s Hikurangi Margin (east coast of the North Island). Over the last decade, conventional petroleum exploration interests in this region have led to the collection of several regional seismic datasets. These data have greatly improved our understanding of hydrate accumulations in the area; however, the resolution of industry multichannel seismic surveys is limited by the bandwidth of the airgun sources used. We present preliminary results from an academic high-resolution generator-injector (GI) airgun seismic survey, undertaken in mid 2015, that targeted focused gas hydrate accumulations lying within thrusted accretionary units in the Pegasus Basin, at the south end of the Hikurangi Margin. Each feature was surveyed with 5 to 10 closely spaced seismic lines that provide an opportunity to examine the three-dimensional structure and stratigraphy with better resolution than the original lines. Two main processes of natural gas migration and resulting accumulation as hydrate are examined more fully: (1) vertical transport into the shallow seafloor driven by overpressure and (2) inclined transport upward along dipping permeable beds. In both of these cases, our data show significant three-dimensional variability is needed to focus fluid migration from below into hydrate trapping configurations within the hydrate stability field nearer to the seafloor. Due to the absence of well data in this basin, the high-resolution seismic data also help to constrain interpretations of basin stratigraphy which plays a significant role in hosting and trapping hydrate accumulations.

Prolonged dynamic support from the Icelandic plume of the NE Atlantic margin

Sedimentary basins affected by hotspots often contain records of uplift and subsidence within coeval stratigraphic successions. The subsidence history can contain measurable perturbations in ancient palaeogeographies that can help constrain the duration of dynamic support. At c. 56 Ma the NE Atlantic experienced uplift related to the Iceland mantle plume. Within the Faroe–Shetland Basin, we document the stratigraphic record of subsidence following plume uplift, through integration of regional seismic datasets and well data. Subsidence following plume uplift is recorded by mapping the southward migration of palaeocoastlines throughout the early Eocene of the Faroe–Shetland Basin. We find that after initial uplift over 0.5 myr, subsidence was inhibited for 0.45 myr. Coeval with initiation of rifting in the North Atlantic, at 54.9 Ma, a c. 0.9 myr period of accelerated subsidence occurred, recorded by migration of the coastline by c. 80 km inland. We attribute these events to a prolonged period (2 myr) of dynamic support from the Iceland plume terminated by rapid loss of dynamic support coeval with rifting in the NE Atlantic at 54.9 Ma. Our results suggest that palaeogeographical analysis is a powerful tool in constraining the duration of dynamic support in basins affected by mantle plumes.

Re-evaluation of the Mentelle Basin, a polyphase rifted margin basin, offshore southwest Australia: new insights from integrated regional seismic datasets

Abstract. Vintage 2-D (two-dimensional) seismic reflection surveys from the sparsely explored Mentelle Basin (western Australian margin) have been reprocessed and integrated with a recent high-quality 2-D seismic survey and stratigraphic borehole data. Interpretation of these data sets allows the internal geometry of the Mentelle Basin fill and depositional history to be reanalysed and new insights into its formation revealed. Basin stratigraphy can be subdivided into several seismically defined megasequences separated by major unconformities related to both breakup between India-Madagascar and Australia-Antarctica in the Valanginian-Late Hauterivian and tectonically-driven switches in deposition through the Albian. Resting on the Valanginian-Late Hauterivian breakup unconformity are several kilometre-scale mounded structures that formed during Late Jurassic to Early Cretaceous extension. These have previously been interpreted as volcanic edifices although direct evidence of volcanic feeder systems is lacking. An alternative interpretation is that these features may be carbonate build-ups. The latter interpretation carries significant climatic ramifications since carbonate build-ups would have formed at high palaeolatitude, ~60° S. Soon after breakup, initial subsidence resulted in a shallow marine environment and deposition of Barremian-Aptian silty-sandy mudstones. As subsidence continued, thick successions of Albian ferruginous black clays were deposited. Internally, seismic megasequences composed of successions of black clays show previously unresolved unconformities, onlapping and downlapping packages, which reflect a complex depositional, rifting and subsidence history at odds with their previous interpretation as open marine sediments. Southwestwards migration of the Kerguelen hotspot led to thermal contraction and subsidence to the present day water depth (~3000 m). This was accompanied by Turonian-Santonian deposition of massive chalk beds, which are unconformably overlain by pelagic Palaeocene-Holocene sediments. This substantial unconformity is related to the diachronous breakup and onset of slow spreading between Australia and Antarctica, which may have led to the reactivation and inversion of basement faults and was followed by rapid seafloor spreading from the Middle Eocene to the present.

Prospectivity and seismic expressions of pre- and post-salt plays along the conjugate margins of Brazil, Angola and Gabon.

The conjugate margins of Brazil and West Africa represent a prolific hydrocarbon province with proven potential in a variety of plays. The application of conjugate margin reconstruction coupled with regional seismic datasets enables the characterization and identification of a range of new structural and stratigraphic plays. This discussion will focus upon the potential of pre-salt and post-salt Albian plays across the Santos, Campos, Kwanza, Congo and South Gabon basins.

Structural controls on Witwatersrand gold mineralisation

Structural controls on the distribution of gold in the Witwatersrand Basin can be seen at scales ranging from that of thin sections up to regional seismic data sets. At the thin section scale, gold occurs largely in fractures with hydrocarbons. These fractures are associated with pulses of thrusting deformation that occurred in the latter stages of the basin's development. The distribution of thrust–fracture networks (and therefore gold) is controlled by the mechanical attributes of the host stratigraphy. Thrust displacements are generally very low, with structures exploiting depositional contacts and mesoscale sedimentary bedforms. Many of the small faults are isolated from each other with bimodal vergence between larger-scale thrusts of the same age. Early-formed faults and fractures become folded and faulted by subsequent propagation of larger-scale thrusts. These characteristics are consistent with formation of the mineralised thrust–fracture networks in the frontal, low-displacement parts of a thrust system. In the Carletonville goldfield, the prospectivity of the Ventersdorp Contact Reef (VCR) has been evaluated by using underground observations to pinpoint the structural habitat of gold, and therefore characterise the size of prospective zones around structures intersecting the VCR, mapped within 3D seismic data. In the Welkom goldfield, the controls on gold distribution at the Tshepong mine have been characterised from 3D seismic data, drilling and underground observations, and used to develop an ore-body model that aids reserve estimation and production planning.