Abstract The Istria ‘Depression’ or sub-basin of offshore Romania lies at the intersection of the trans-European Tornquist–Teisseyre ‘Zone’ and the Black Sea back-arc basin, just outboard of the East Carpathian orogenic welt. Its Late Mesozoic–Cenozoic succession records an extraordinary polyphase history of subsidence and sedimentation, interrupted by several quite spectacular second-/third-order erosional unconformities, reflecting the interplay between these tectonic domains. The unconformities divide the succession into a number of stratigraphic sequences. The sub-basin first developed as a transtensional rift in the Triassic–Early Jurassic, evolving into a narrow oceanized trough in the later Jurassic. This was tilted west during the Early Cretaceous, and the residual Late Jurassic topography was filled and buried by a west-facing clastic–evaporite wedge. Following Late Aptian–Albian(?) rifting, post-rift subsidence and spreading in the Western Black Sea imposed a strong easterly tilt, encouraging the partial evacuation of its Early Cretaceous sedimentary fill by gravity-driven mass wastage. The incised valley topography was subsequently infilled and buried during the later Cretaceous and Early Cenozoic. During the mid-Late Cenozoic, the Black Sea Basin experienced intermittent periods of partial to complete isolation from the world ocean and significant base-level drawdown. The first major sea-level fall occurred in the Eocene when the Istria ‘Depression’ was deeply incised, to be healed by Oligocene shales during the subsequent rise. Yet another period of drawdown and exposure occurred in the mid-Miocene, with extensive shelf-margin mass wastage and erosion, followed by re-flooding and deposition of a transgressive backstepping sequence in the middle-late Miocene. Messinian drawdown in the Mediterranean caused a further period of isolation and falling base level. The shelf margin was again exposed, and experienced widespread mass wastage and slumping. Rising sea level eroded the earlier slumped sequence and the margin was healed by a lowstand prograding wedge in the late Miocene–early Pliocene. This was followed by shelf sedimentation in the Plio-Pleistocene periodically interrupted by canyon-incision events, testifying to continued climatically or tectonically imposed base-level fluctuations. Several direct and indirect tectonic factors were responsible for valley/canyon incision within the Istria Depression and erosion of the Romanian Black Sea shelf margin. These include: (1) the local structural framework; (2) direct tectonic uplift and tilting; and (3) more indirect tectonically imposed isolation encouraging significant base-level falls.

ABSTRACT The Jansz-Io gas field is located in production licenses WA-36-L, WA-39-L, and WA-40-L within the Carnarvon Basin, northwest shelf, Australia. It is 70 km (43 mi) northwest of the Gorgon gas field, 140 km (87 mi) northwest of Barrow Island, and 250 km (155 mi) from Dampier on the northwest coast of Western Australia. Water depths vary from 1200 to 1400 m (3937 to 4593 ft) across the field. The Jansz-Io gas field was discovered in 2000 by the Jansz-1 exploration well. A three-dimensional (3-D) seismic survey was acquired in 2004, and a further five wells were drilled between 2000 and 2009 to further delineate the field extent and size and characterize the resource to facilitate progress toward development. The Jansz-Io hydrocarbon trap extends over 2000 km 2 (772 mi 2 ) with both structural (faulted anticline) and stratigraphic (reservoir pinch-out) components. The stratigraphic component of the trap is defined by the reservoir extent, which is limited by depositional downlap to the northwest, and erosional truncation by Upper Jurassic and Lower Cretaceous unconformities to the southeast. The reservoir comprises muddy, bioturbated, predominantly very fine- to fine-grained sandstones deposited in a shallow-marine environment and is divided into two units. The upper wedge reservoir has 25 to 35% total porosity with 10 to 1000 md permeability, and the lower wedge reservoir has 15 to 25% porosity with 0.01 to 10 md permeability. Both reservoir units are expected to contribute gas during production. The original gas in place (OGIP) for the Jansz-Io Oxfordian reservoir has a probabilistic range from 320 to 946 Gm 3 (11 to 33 tcf), with a P50 value of 632 Gm 3 (22 tcf). The ultimate recovered gas for the field will depend on both the development plan and the reservoir performance over field life. For the current 15-well development plan, the resource estimates range from 201 to 442 Gm 3 (7 to 16 tcf). The Jansz-Io gas field is a key part of the greater Gorgon liquified natural gas (LNG) project and will supply gas to the LNG plant that is being constructed on Barrow Island. The development concept includes subsea completions from three drill centers placed on the seafloor connected to a subsea production pipeline to carry gas to the LNG processing plant. For the first stage of field development, 10 development wells were successfully drilled and completed during 2012 and 2014. The second drilling campaign is planned to commence after field start-up with the timing dependant on field performance.

Abstract As hydrocarbon-prone basins mature through time, stratigraphic traps become increasingly important as hosts for yet-to-find reserves. Explorationists strive to reduce the uncertainty in reservoir distribution and quality, but considerable complications exist in the evaluation of stratigraphic traps owing to the inextricable links between stratigraphy, trap definition and their subsequent risking. This study quantifies the relationships that exist between reservoir geometries and the rates of reservoir property degradation in a turbidite sandstone pinch-out zone. The investigation focuses on the Paleocene Forties Sandstone Member of the Everest and Arran fields of the East Central Graben of the UK North Sea. We utilized standard seismic interpretation techniques and integrated stratigraphic and petrophysical analysis of wireline log data to map deep-water turbidite sandstone terminations and develop a predictive model for reservoir property changes close to the feather edge. The Forties Sandstone Member thins systematically up on to a palaeoramp on the eastern basin margin of the Central Graben. Results reveal that the net reservoir sandstones pinch out after the turbidite flows had traversed 5 km across the palaeoramp. The gross interval is predicted to completely terminate 6.4 km up the palaeoramp. The reservoir properties decrease in concert with the thinning trend in the wedge zone as a function of the interaction of palaeotopography and the hydraulics of the decelerating flows. The inclination of the counter-regional slope is considered to be a key controlling factor that determines the rate of thinning and thus the termination position of the sandstones and their concomitant reservoir property decline. The results of this study demonstrate that characterization of pinch-outs into distinct zones based on a palaeotopographic template can be of utility in stratigraphic and combination trap definition. This work also has wider implications for prospect risking, volumetric analysis, the population of properties and geological modelling of stratigraphic traps.

This paper presents geophysical and core data obtained from several marine geology surveys carried out in the western Black Sea. These data provide a solid record of water-level fluctuation during the Last Glacial Maximum in the Black Sea. A Last Glacial Maximum lowstand wedge evidenced at the shelf edge in Romania, Bulgaria, and Turkey represents the starting point of this record. Then, a first transgressive system is identified as the Danube prodelta built under ~40 m of water depth. The related rise in water level is interpreted to have been caused by an increase in water provided to the Black Sea by the melting of the ice after 18,000 yr B.P., drained by the largest European rivers (Danube, Dnieper, Dniester). Subsequently, the Black Sea lacustrine shelf deposits formed a significant basinward-prograding wedge system, interpreted as forced regression system tracts. On top of these prograding sequences, there is a set of sand dunes that delineates a wave-cut terrace-like feature around the isobath −100 m. The upper part of the last prograding sequence is incised by anastomosed channels that end in the Danube (Viteaz) canyon, which are also built on the lacustrine prograding wedge. Overlying this succession, there is a shelfwide unconformity visible in very high-resolution seismic-reflection profiles and present all over the shelf. A uniform drape of marine sediment above the unconformity is present all over the continental shelf with practically the same thickness over nearby elevations and depressions. This mud drape represents the last stage of the Black Sea water-level fluctuation and is set after the reconnection of this basin with the Mediterranean Sea.