Abstract This is a case study of a 3-D anisotropic prestack depth migration (APSDM) of data from Block L1.0 of the Dutch sector of the North Sea. Producing gas reservoirs in L10 are typical of the area in that they are contained in horst and tilted fault blocks of the Rotliegend Formation. Imaging these fault blocks is made difficult by a complex overburden greatly influenced by salt tectonics. in particular, the overburden includes chalk characterized by fast velocities and a strong vertical velocity gradient. Prestack depth migration (PSDM) is often used in this ara,but the resulting depth images are considered mac-citrate because anisotropy is ignored. Indeed, these depth images are routinely calibrated to well depths, but this is difficult when wells are highly deviated. By contrast, APSDM allows the use of the depths of formation tops from all wells, including highly deviated wells, to constrain and enhance the velocity model. Therefore, accurate depths can be obtained directly from the APSDM image. APSDM should also provide the true lateral positions of dipping events and fault terminations in addition to better focusing of the image To perform APSDM, the anisotropy parameters for the geology must first be estimated, and this task is the main topic of this article. In this study, the parameters were derived from vertical check-shot data, sonic logs, and surface seismic data.

Abstract Porosity-depth plots for sandstones commonly show a decrease in porosity with depth. In more deeply buried sandstones, this reflects an increase in the amount of mineral cement. This correlation is often thought to indicate a causal relationship; however, in this paper, we examine data that argue against a direct depth control on cementation. In sandstones from the Garn Formation, Hal- tenbanken, the homogenization temperatures of primary fluid inclusions in quartz cement are correlated with depth over the entire basin. The inclusions do not appear to have stretched or leaked so the correlation suggests a widespread cementation event affecting large parts of the basin at about the same time. K-Ar ages of illite cements from eolian sandstones of the Rotliegend Formation (Village Fields area, southern North Sea) all fall within the Middle to Late Jurassic and show no correlation with depth at time of cementation. There is, however, a weak correlation between illite oxygen isotope ratio and depth at time of illite growth. Both ages and isotope ratios again suggest a widespread cementation phase taking place within a restricted time period. Published data from other sandstones, including the Brent sandstone reservoirs of the North Sea, tell a similar story, indicating cementation at a particular time rather than temperature or depth. In the case of quartz, cementation seems to coincide with rapid subsidence and heating, while in the Southern North Sea, illite growth is approximately synchronous with rifting and consequent rapid subsidence and heating. These effects may have influenced fluid flow and/or precipitation mechanisms in such a way that regional cementation occurred.

Abstract The Slochteren No. 1 well discovered in 1959 what now is known as the Groningen gas field in the northern Netherlands. The field is on a culmination of the large, regional northern Netherlands high which was formed during the late Kimmerian tectonic phase (Late Jurassic-Early Cretaceous). However, there is some evidence that the structure was a positive element during earlier periods, i.e., during Triassic and possibly even late Carboniferous times. The reservoir overlies unconformably the truncated and strongly faulted coal-bearing Pennsylvanian strata which are considered to be the main source of the Groningen gas. The reservoir, 300-700 ft thick, consists of fluviatile and eolian sandstone and conglomerate of the Rotliegendes Formation (Lower Permian). These coarse clastic beds are overlain by a few thousand feet of Permian Zechstein evaporites, notably rock salt and to a lesser extent anhydrite and dolomite, which constitute the very effective reservoir seal. Because of intensive salt movements, the thickness of the overlying Mesozoic and Cenozoic strata ranges from 3,000 ft to more than 6,500 ft. The field covers 180,000 acres, and the reserves now are estimated at 58 trillion ft 8 . Present production potential is 2 billion ft 3 of gas per day from nine “clusters” of about six closely spaced wells each. The favorable reservoir properties of the sandstone allow, at least for the present, drainage of the field from the structurally highest southern part.

ABSTRACT The environments of deposition of the Upper Rotliegend in the Southern Permian Basin have been of continuing interest since the discovery of the Groningen Gas Field in 1959. This is because the quality of the porosity and permeability of the sands is in most places dependent upon the original depositional environments. In the earliest years, the emphasis of much work was on mapping broad depositional realms of aeolian, fluvial, and playa environments. As new discoveries came under development, the geometry of small-scale features such as dune (good reservoir) and interdune (poor reservoir) became important, informed in part by studies of modern deserts. As the Rotliegend play matured, the emphasis of studies shifted to approaches that recognised rock units below the level of the known members of the Rotliegend that were tied into process framework elements such as climate change. The present study uses modern analogues for the Rotliegend at Bristol Dry Lake and Cadiz Dry Lake, in the California desert, U.S.A., as a basis for the discussion of ancient Rotliegend depositional environments. At these sites active aeolian, fluvial, and playa depositional environments have produced a sedimentary record similar to that in the Rotliegend. Additionally, there exists at Bristol Dry Lake a sand-flat sedimentary domain that constitutes a transition zone between the playa and surrounding fluvial and aeolian sedimentation, where the processes associated with all three major environments are roughly in balance. A sand flat also exists in the ancient Rotliegend, although it is more commonly referred to as the “transition zone” in North Sea literature. At Cadiz Dry Lake the geographical resemblance to the Rotliegend is impressive, with winds blowing parallel to the elongation of the playa and ephemeral streams entering from the margins in a manner analogous to the Rotliegend of the Southern North Sea, where fluvial and wind transport directions are at roughly right angles in much of the basin. Our work suggests that in our study area the Silverpit Formation has the qualities of a desert playa lake that was episodically flooded. We illustrate here with core examples the major lithofacies of the Rotliegend, including aeolian, fluvial, and playa sediments. These are tied to a stratigraphic scheme that is based on the major formations of Upper and Lower Slochteren, and a new, if minor, member known as the Hyde Sandstone, representing the last “regression” of the playa in the UK. Isopach and palaeogeographic maps of the major sedimentary domains that are dominated by aeolian, fluvial, or playa processes illustrate the evolution of the Rotliegend. The maps show the irregularity of the Sand Flat and its lateral shifts through time, as well as the distribution of the other domains due to dominance of wind or fluvial sedimentation locally. They also illustrate the dependence of much of the facies distribution on the palaeotopography of the Carboniferous. The development of dune fields that became the best reservoir rock was dependent upon the location of fluvial systems that supplied sand to the wind, and upon topographic slope. Much of the best dune reservoir was deposited not in the basin centres, where it became deeply buried and compacted, but on the flanks of major structures as windward and leeward sand seas and dune fields.