Structural trap

Abstract: Jonah field, Wyoming's second largest gas producer, is a structurally controlled trap located in the northwestern part of the Green River basin. Gas and condensate are produced from innumerable latest Cretaceous and early Tertiary overpressured tight-gas sandstones at depths of 7300–12,800 ft (2200–3900 m). Jonah field is remarkable for many reasons, including the large per-well reserves (relative to other tight-gas reservoirs), hundreds of feet of net pay, and a gross producing interval as great as 4000 ft (1220 m) thick. These superlative production characteristics exist, despite the fact that the structural trap is subtle and locally cryptic. Advanced seismic techniques define the Jonah trap boundaries and add value when they are used to position wells in proximity to the faults and on subtle structures. One of the main seismic techniques is a three-dimensional, broadband, amplitude-based coherency algorithm that has edge-detection capabilities. This algorithm analyzes the reflector amplitude gradient and records the lateral change in amplitude with azimuthal angle, which allows the interpreter to illuminate a particular feature from the optimal angle to reveal the maximum detail in the data. Gas entrapment at Jonah field is enabled by two bounding faults. Fault throw is variable but commonly less than 200 ft (60 m), and the major faults are nearly vertical. Two field-bounding fault zones, the west fault and south Jonah fault, intersect updip toward the southwest to create the overall wedge-shaped trap. The updip edge of a tilted fault block underlies the prolific Stud Horse Butte anticline and the Cabrito nose trends. The principal in-field faults terminate at the south Jonah fault to form four compartments, each comprised of a northeast-plunging, faulted nose or homocline bounded on the west and south by faults. The Jonah faults juxtapose high- and low-reserve wells; high-reserve wells are concentrated on the east side of the north- and northeast-trending faults, regardless of their sense of displacement. The south Jonah fault is probably a left-lateral, wrench-fault zone. The south Jonah fault was active concurrently with Lance Formation deposition, resulting in thicker Lance north of the fault in the center of the field and thin along the updip edge of the tilted block. Subsequent post-Paleocene motion on the fault caused formation of the Stud Horse Butte anticline, which is evident in the basal Tertiary strata. Jonah is a multipay field with numerous productive lenticular, fluvial sandstones. Whereas local structural features control the Jonah trap, regional structural elements shaped the fluvial system that deposited the reservoir sandstones. Regional structural features also affected the burial history that resulted in petroleum generation. Gas isotope composition and the thermal maturity of the producing strata indicate that the field produces hydrocarbons formed in deeper strata. The gas field shows evidence of paleostructural growth, which, in combination with the production pattern along the faults and the bottoms-up origin of the gas, may explain the enigmatic charging of these low-permeability sandstone bodies according to structural position, i.e., segregation by buoyancy. In this scenario, gas emplacement occurred before the reduction of sandstone permeability to the present condition.

Abstract: In low permeability-tight reservoir,fluid filtration belongs to the non-Darcy flow. Only when the pressure gradient comes to the starting pressure gradient,can the filtration occur. Overpressure is a predominant dynamic force for hydrocarbon migration and accumulation in low permeability-tight reservoir,and "dynamic trap" is the most important action for hydrocarbons overpressure-charged into low permeability-tight reservoirs to form accumulations and also a three-dimensional space for hydrocarbons to be relented and accumulated. "Dynamic trap" is quite different from the conventional trap with mid-high permeability reservoir such as structural trap,stratigraphic trap and lithologic trap. The difference shows not only in charging dynamic force,filtration way,but also in oil-gas-water relation,trap shape and distribution. Advantages to form a "dynamic trap" are believed as sources and reservoirs are superimposed or interbedded in large scale,overpressures of hydrocarbon generation can act on reservoirs next to sources and hydrocarbons migrate in short distance in a pore-fracture network,which finally lead to the formation of large-scale continuous hydrocarbon-bearing interval. The "dynamic trap" ,located in the superimposition part of the source "sweet point" and the reservoir "sweet point" ,is the most easliy to form unconventional reservoirs. The presentation of the "dynamic trap" concept hopes to supply new thinking for enriching petroleum geology theory,deepening hydrocarbon accumulation's classification and prospecting unconventional hydrocarbon accumulations.

Abstract: The geological history of Germany’s largest and most productive petroleum accumulation, the Mittelplate oil field in Schleswig-Holstein (Northern Germany), is reconstructed by simulating the structural and thermal evolution along a 2D cross-section. The Mittelplate field is located at the western flank of the Busum salt dome at the transition from the Schleswig-Holstein mainland to the German North Sea Sector. Organic geochemical data confirm the Lower Jurassic Posidonia Shale to be the predominant oil source rock in the Schleswig-Holstein area. The studied section is characterized by salt walls and salt domes built up by Permian evaporites. Reconstruction of the structural and thermal evolution of the Mittelplate field by means of basin modelling reveals the dominating influence of salt dynamics on the entire petroleum system: The development of secondary rim-synclines during salt rise provided accommodation space for the deposition of the Posidonia Shale as well as the deltaic Middle Jurassic reservoir sandstones. The rise of the nearby Oldenswort salt wall controlled the timing of maturation and petroleum generation during Cenozoic times. Hydrocarbon migration from the Posidonia Shale into the reservoirs occurred up-dip from the deeper subsiding rim-syncline into the structural trap with the Middle Jurassic reservoir sandstones pinching out at the flank of the Busum salt dome. Along the modelled 2D section the field’s recent temperature field and its complex reservoir architecture are reconstructed.

Abstract: Timor island, situated in the Banda arc-continent collision zone, is the location of numerous oil and gas seeps, and contains high-quality hydrocarbon source rocks. Despite this, the island is widely considered to have only moderate petroleum potential due to its well known structural complexity. It is suggested here, however, that this complexity is limited to shallow structural levels, and below this a simpler structural style predominates. In particular, the Kolbano area of southern West Timor is interpreted to be underlain by a very large and structurally simple inversion anticline, capable of hosting a giant hydrocarbon accumulation. The Banli-1 exploration well only penetrated the flank of this structure, perhaps 300-500m below the prospective crest. Banli-1 was drilled on a gravity high, which corresponds to the footwall basement block of the inversion structure, not to the crest of the hangingwall anticline. Similar if smaller structures are likely to occur elsewhere both onshore in West Timor and offshore to the south in the northern slope of the Timor Trough.