PG1085 Toward a quantitative definition of mechanical units: New techniques and results from an outcropping deep-water turbidite succession (Tanqua-Karoo Basin, South Africa)The physical properties of reservoirs are strongly influenced bydistributed fracture fields. Outcrop studies are commonly used todetermine them but have provided unsatisfactory results becausethe definition of mechanical units, i.e., (groups of) layers displayinghomogeneous fracture patterns, is difficult and typically conductedin a qualitative manner. We have developed a systematicmethodology to acquire and process fracture patterns in outcropsand to define their relation with stratigraphy. We dedicate particularattention to the vertical distribution of fractures in a sedimentarysuccession, commonly composed of layers of different thicknessesand compositions. The method makes full use of geographic informationsystem technologies and allows for direct digital acquisitionin the field leading to time-efficient acquisition. Data areprocessed with a newly developed routine that permits an objectivedescription of the changes of fracture characteristics along the stratigraphyof the outcrop. The operator is then able to define the mostsuitable fracture stratigraphy. The integration of results from differentoutcrops is thought to provide a tool for predicting fracturedistributions in subsurface target areas.
PG1099 Geologic controls on transgressive-regressive cycles in the upper Pictured Cliffs Sandstone and coal geometry in the lower Fruitland Formation, northern San Juan Basin, New Mexico and ColoradoThree upper Pictured Cliffs Sandstone tongues in the northern part of the San Juan Basin record high-frequency transgressive episodesduring the Late Cretaceous and are inferred to have been caused byeustatic sea level rise coincident with differential subsidence. Outcropand subsurface studies show that each tongue is an amalgamatedbarrier strand-plain unit up to 100 ft (30 m) thick. Successiveupper Pictured Cliffs tongues display an imbricate relation and areoffset basinward, reflecting net shoreline progradation northeastward.Upper Pictured Cliffs barrier strand-plain sandstones underlieand bound thickest Fruitland coal seams on the seaward side. Controlson Fruitland coal-seam thickness and continuity are a functionof local facies distribution in a coastal-plain setting, shoreline positionsrelated to transgressive-regressive cycles, and basin subsidence.During periods of relative sea level rise, the Pictured Cliffs shorelinewas temporarily stabilized, allowing thick, coastal-plain peats to accumulate.Although some coal seams in the lower Fruitland tongueoverride abandoned Pictured Cliffs shoreline deposits, many pinchout against them. Differences in the degree of continuity of thesecoal seams relative to coeval shoreline sandstones are attributed to either differential subsidence in the northern part ofthe basin, multiple episodes of sea level rise, local variationsin accommodation and progradation, stabilizationof the shoreline by aggrading peat deposits, or a combinationof these factors. Fruitland coalbed methane resourcesand productivity are partly controlled by coalseamthickness; other important factors include thermalmaturity, fracturing, and overpressuring. The dominantproduction trend occurs in the northern part of thebasin and is oriented northwestward, coinciding withthe greatest Fruitland net coal thickness. Similar relationshipsbetween trends of thick coal seams of coevalorigin with stacked shoreface sandstones exist in otherWestern Interior basins in the United States and serveas models for coalbed methane exploration in otherbasins of the world.
PG1123 Overpressure generation and evolution in a compressional tectonic setting, the southern margin of Junggar Basin, northwestern ChinaOverpressure is widespread in the southern margin of the Junggar Basin, northwestern China. Pressure measurements in drillstem tests and repeated formation tests and estimates from wire-line logs indicate contrasting overpressure values between permeable sandstones and adjacent low-permeability mudrocks. In addition, excess pressure differs among anticlines with similar depth, lithologies, and geologic age, indicating significant lateral changes of overpressure. Major factors controlling overpressure generation and distribution include rapid sediment deposition, pressure compartmentalization by thick mudrocks, tectonic stress, faulting, and folding. Clay transformation and hydrocarbon generation are believed to be insignificant in overpressure generation in the southern Junggar Basin. Numerical modeling of pressure generation and evolution suggests that faulting and stratal tilting associated with folding are the most significant factors in the overpressure generation of a permeable sandstone.The extremely high overpressure (pressure coefficient up to2.43) may have been caused by hydraulic adjustment within permeable sandstones associated with structural deformation caused bypost-Miocene intense tectonic activities.
PG1141 Seal bypass systemsWe present an interpretational framework for the analysis of a diverse set of geological structures that breach sealing sequences and allow fluids to flow vertically or subvertically across the seal. In so doing, they act as seal bypass systems (SBS). We define SBS as seismically resolvable geological features embedded within sealing sequences that promote cross-stratal fluid migration and allow fluids to bypass the pore network. If such bypass systems exist within a given seal sequence, then predictions of sealing capacity based exclusively on the flow properties (capillary entry pressure and hydraulic conductivity) of the bulk rock can potentially be negated by the capacity of the bypass system to breach the grain and pore network. We present a range of examples of SBS affecting contrasting types of sealing sequences using three-dimensional (3-D) seismic data. These examples show direct evidence of highly focused vertical or subvertical fluid flow from subsurface reservoirs up through the seal sequence, with leakage internally at higher levels or to the surface as seeps. We classify SBS into three main groups based on seismic interpretational criteria: (1) fault related, (2) intrusion related, and (3) pipe related. We show how each group exhibits different modes of behavior with different scaling relationships between flux and dimensions and different short- and long-term impacts on seal behavior.
PG1167 Petroleum systems in the offshore Xihu Basin on the continental shelf of the East China SeaXihu Basin is one of the Cenozoic sedimentary basins within the continental shelf of the East China Sea, within which eight oil and gas fields and four hydrocarbon-bearing structures have been found. Our systematic analysis of potential petroleum systems in the basin has identified the Eocene Pinghu Formation as the most important source rock in the basin. The Eocene Pinghu Formation consists of mudstone and coal deposited in an embayment and tidal-flat environment and is characterized as containing type III kerogen. The Pinghu Formation is mature and, at the present time, is within the oil and wet-gas windows with determined vitrinite reflectance values in the range of 0.55–2.2% measured vitrinite reflectance (Rom). Modeling results suggest that the main stage of hydrocarbon expulsion occurred during the Miocene. The main reservoir consists of fine-grained sandstones of the Huagang and Pinghu formations deposited within shallow lacustrine and estuarine environments. The measured porosity from core samples of reservoir rock ranges from 10 to 35%, whereas permeability values range from 1 to 4000 md; both porosity and permeability decrease with depth. The mudstones of the upper Huagang and Longjing formations also occur as regional seals, which developed overpressure zones as determined by two-way sonic transit times. The overburden includes all the Oligocene, Miocene, Pliocene, and Quaternary strata. The hydrocarbon traps in the basin are mainly structural traps, including faulted blocks, faulted structural noses, and anticlines. Most traps were developed at the ends of the Eocene or Oligocene. Hydrocarbons produced fromthe Xihu Basin are predominantly natural gases with minor amounts of crude oil. The crude oil in the basin is characterized by a low density, low sulfur, low wax, low vanadium/nickel ratio, a low resin and asphaltene content, and a high proportion of saturated hydrocarbons. The natural gas in the basin is composed predominantly of methane, with an average C1/PC1–5 ratio of 0.89 and a C1/PC2–5 ratio of 8.6. Two petroleum systems have been identified in the basin: the known Pinghu-Huagang and the hypothetical Huagang-Huagang systems. The Pinghu-Huagang is the major petroleum system and most important for oil and natural gas exploration in the basin.
PG1189 Lower Paleozoic petroleum from southern Scandinavia: Implications to a Paleozoic petroleum system offshore southern NorwayPetroleum occurring in lower Paleozoic rocks is known to be present in southern Scandinavia, northern Poland, and the Baltic states. Oil has been produced from lower Paleozoic reservoirs in Sweden; northern Poland; and the Baltic countries Lithuania, Latvia, and the Russian exclave area of Kaliningrad. The sources for this petroleum are marine, organic-rich muds deposited in the Cambrian, Ordovician, and Silurian. This article concerns geochemical analysis of oils extracted from sandstones and carbonates from the Norwegian Oslo Graben rift and locations in Sweden and describes, in addition, insoluble bitumens collected from lower Paleozoic rocks in the Oslo Graben, locations in Sweden, and from upper Paleozoic rocks in a Norwegian North Sea well. The oils in this study have several geochemical characteristics shared with oils from the Baltic states and northern Poland, and the maturities of the oils are, in general, low. The occurrences of bitumen and migrated petroleum in the Oslo Graben lead us to believe that petroleumalso has been generated and expelled in the related offshore Skagerrak Graben, indicating that a Paleozoic petroleum system operated in the Skagerrak Graben. This potential petroleum system has not suffered the degree of uplift, erosion, and destruction of reservoirs experienced by the onshore Oslo Graben, making preservation of any petroleum accumulations in the SkagerrakGrabenmore plausible. Although speculative, these considerations should interest anyone involved in petroleum exploration in the Skagerrak and the Norwegian-Danish Basin, not the least because of the proximity of Skagerrak and major energy markets in Europe.
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