Seals For Hydrocarbon Accumulations Research Articles

Emplacement dynamics of a complex thick mafic intrusion revealed by borehole image log facies analyses: Implications for fluid migration in the Parnaíba Basin petroleum system, Brazil

Sill intrusions may play a significant role in the development of petroleum systems in sedimentary basins by promoting maturation of source rocks, influencing fluid migration and acting as reservoirs, traps, and seals for hydrocarbon accumulations. Within this study, we present a novel approach to interpreting geophysical borehole data for a ca. 174 m thick sill from the Parnaíba Basin revealing new insights into both the emplacement mechanisms and impacts of the sill on hydrocarbon migration and trapping. High-resolution ultrasonic and microresistivity borehole image logs were integrated with conventional log data, petrography of eight side wall cores and total gas data recorded during drilling. The petrographic analysis revealed finer crystalline dolerite at the top and bottom layers of the sill while the upper central portion revealed coarser crystalline gabbroic facies, associated with anomalies in gamma ray, neutron porosity, sonic and geochemical log responses. From borehole image logs we interpreted key facies units including massive, fractured, cooling jointed and layered dolerite along with clear evidence for layers containing diverse inclusions interpreted as enclave structures. The integration and correlation of these data with analogue studies allowed the interpretation of gradational cooling with magmatic differentiation, partially melted host rock linked to sulphide production and organic matter maturation, mafic microgranular enclaves related to mingling processes of magma recharge, banding layers related to cumulate zones, and intense cooling fractures in the central sill body. An interval of ca. 44 m at the top of the sill with notably less jointing appears to be responsible for sealing the gas underneath, whilst the fractured central portion of the sill has enabled gas migration evidenced by increased gas signatures. In addition, this work also reveals for the first-time image log evidence for magma mingling and indications of assimilation and anatexis processes within the Parnaíba Basin. This study has important implications for improving the understanding of emplacement mechanisms of thick parallel layer intrusions and their implications for fluid flow in sedimentary basins worldwide.

The petrophysical properties of deformation bands in relation to their microstructure

Deformation bands are significant discontinuities in reservoir sandstones providing baffles to the migration of fluids and acting as seals for hydrocarbon accumulations. Their contribution to these processes is dependent upon a range of inter-related factors including lateral continuity, relative displacement, reservoir geohistories and rock microstructure. Deformation bands have millimetre-scale displacements, which are smaller than the seismic resolution, and hence the study of their microstructure from reservoir cores is a necessary tool in the prediction of their fault seal potential. The latter is only achieved if detailed petrophysical measurements of fault microstructure are carried out using techniques, which are suitable for measurement of very low permeability fault rock at the relevant scale. Conventional techniques of fault rock porosity and permeability determination invite host rock bias in measurement and inevitably underestimate the potential of these structures as fluid barriers. Pressure decay probe permeametry used in this study has the advantage of measuring small volumes of rock at finer spatial resolutions and detailed permeability distributions around common types of deformation bands are obtained. Together with supporting data provided by conventional core analysis techniques, these measurements show that the greatest differences in petrophysical properties relative to host rock occur in those bands, which have experienced cataclasis. These results provide valuable input into reservoir simulation models and help reduce much of the uncertainty regarding the role of different types of deformation bands in fault seal.

Regional Stratigraphy and Petroleum Potential, Ghadames Basin, Algeria

The Ghadames basin in east-central Algeria extends over 65,000 km{sup 2} (25,000 mi{sup 2}), of which 90% is covered by dunes of the eastern Erg. This intracratonic basin consists of up to 6000 m (20,000 ft) of dominantly clastic Paleozoic through Mesozoic strata. The Ghadames basin is part of a larger, composite basin complex (Ilizzi-Ghadames-Triassic basins) where Paleozoic strata have been truncated during a Hercynian erosional event and subsequently overlain by a northward-thickening wedge of Mesozoic sediments. Major reservoir rocks include Triassic sandstones that produce oil, gas, and condensate in the western Ghadames basin, Siluro-Devonian sandstones that produce mostly oil in the shallower Ilizzi basin to the south, and Cambro-Ordovician orthoquartzites that produce oil at Hassi Messaoud to the northwest. Organic shales of the Silurian and Middle-Upper Devonian are considered primary source rocks. Paleozoic shales and Triassic evaporite/red bed sequences act as seals for hydrocarbon accumulations. The central Ghadames basin is underexplored, with less than one wildcat well/1700 km{sup 2} (one well/420,000 ac). Recent Devonian and Triassic oil discoveries below 3500 m (11,500 ft) indicate that deep oil potential exists. Exploration to date has concentrated on structural traps. Subcrop and facies trends indicate that potential for giant stratigraphic or combinationmore » traps exists for both Siluro-Devonian and Triassic intervals. Modern seismic acquisition and processing techniques in high dune areas can be used to successfully identify critical unconformity-bound sequences with significant stratigraphic trap potential. Advances in seismic and drilling technology combined with creative exploration should result in major petroleum discoveries in the Ghadames basin.« less

Evaluating Seals for Hydrocarbon Accumulations: ABSTRACT

Evaluating Seals for Hydrocarbon Accumulations: ABSTRACT

Evaporites as Seals for Hydrocarbon Accumulations in Carbonate Provinces of North America: Case Histories: ABSTRACT

Evaporites are an important factor as the seal for hydrocarbon accumulations in carbonate provinces throughout North America. In the Silurian of the Michigan basin, dolomitized Niagaran pinnacle-reefs have been effectively sealed with evaporite cycles composed of anhydritic sabkha deposits of the Ruff Formation and cleaner evaporites of the Salina units. In the Williston basin of North Dakota and Saskatchewan, successive regressions of the Mississippian sea have resulted in the subsequent basinward migration of anhydritic sabkha deposits over algal-pelletal limestone banks that were formerly shoals at the seaward edge of the sabkhas. The anhydrite has halted the updip migration of hydrocarbons in these limestones. In the Permian of west Texas, over 731 million bbl of oil ave been produced from the San Andres Formation against a porosity barrier along the eastern side of the Central Basin platform where anhydrite has plugged the porosity of the dolomite. The McElroy field is an excellent example of this important trend. The environmental conditions that provide the typical setting for the formation of shallow-marine carbonates are also ideal for the formation of evaporites whenever marine waters become sufficiently restricted. The occurrence of excellent evaporite seals in close proximity to porous carbonate reservoirs can provide many opportunities for entrapment of End_Page 967------------------------------ hydrocarbons in this province. However, a thorough understanding of facies relations is essential for success in the search for this type of subtle stratigraphic trap. End_of_Article - Last_Page 968------------