Abstract Scotland and the adjacent continental shelf have proved to be exceptionally rich in oil and gas thanks to the presence of two source rocks. Onshore, the Carboniferous oil shales around Edinburgh were the basis of a thriving shale oil industry in the latter part of the 19th century. Offshore, the Upper Jurassic Kimmeridge Clay Formation is the source of most of the oil in the North Sea fields, discovered and produced in the latter part of the 20th century. The development of these natural resources has proved to be hugely challenging scientifically, technologically and commercially. The economic return to Scotland and the rest of the United Kingdom has been immense; since 1965 the oil industry has generated an operating surplus of £250 billion (DTI 2000). Hydrocarbons from the UK Continental Shelf as a whole contributed £4 billion to the UK’s gross domestic product in 1999, equivalent to 1.8% of the total. However, in addition to this most obvious and important impact, the production of oil and gas from the ‘geology of Scotland’ has left its mark in other ways. There have been changes in the science of geology as practised in Scotland, and on the history and landscape. In West Lothian the working of oil shale during the late 19th and early 20th century produced the distinctive waste heaps known as bings (see Chapter 20 , Fig. 20.1 ). More recently oil development has brought economic benefits to the northeast of Scotland and the Shetland Islands. This account of

Abstract In the early days of Gulf of Mexico piston coring, locations were chosen on a grid basis or selected from loose 2D seismic surveys. Such locations resulted in some seepage “hits,” but the majority had either a background signature or an “anomalous” value that was between a true visible seep and background (using fluorescence intensity and unresolved complex mixture content). A scale based on these early data identified anything <5,000-10,000 fluorescence units as background, and those values >10,000 were classified as anomalies to seepage depending on individual company interpretations. With the advent of 3D surveys, it is easier to locate seepage-related seabed features using seafloor amplitude extractions related to migration conduits, such as faults associated with shallow salt features or deep-seated faults. As seepage sites are now better defined and as we have an extensive geochemical database, the old scale for background versus anomaly versus seepage has changed. By correlating true seepage to reservoired oil, most “anomalies” are not related to seepage or to the reservoired oils, and therefore, are not related to the subsurface petroleum system. The biomarker signatures can be used to define source origins, and when merged with regional understanding of source rocks in the greater Gulf of Mexico basin, a deep water source model can be derived. A 2D TemisPack model confirms the seepage results based on a deep water source rock model placing the primary source centered on the Tithonian and possible secondary source rocks at the Mid-Cretaceous Unconformity (MCU) and Oxfordian levels. Based on oil-to-seep correlations, we can demonstrate: That most piston cores <30,000 fluorescence represent background, 30,000–50,000 are low confidence anomalies, 50,000–100,000 are high confidence anomalies, and >100,000 are truthable seepage. Biomarker signatures of most piston core extracts having <50,000 fluorescence do not correlate to the reservoired oils; however, the number of cores that may correlate to seepage varies regionally. Fewer piston core extracts correlate to seepage in the eastern and central Gulf of Mexico, whereas more extracts correlate to seepage in the western Gulf of Mexico in the 30,000–50,000 fluorescence range. The effects of recent organic matter contamination also differ in other basins, but its effects remain the same, just the thresholds for truthing the extracts are different. Geographical differences exist. A pervasive background biomarker signature is present across the Gulf of Mexico, related to either river discharge sediments containing extractable oil and/or organic matter, or possible sediment dewatering carrying an oil-like signature, unrelated to the subsurface petroleum system. There is a distinct pattern related to the Mississippi Fan. The “background signatures” appear to contain real oil, but do not correlate to the active true seepage. Using a rigorous approach when interpreting the detailed geochemical data from the piston cores, the “clean” seepage shows a regional trend that can be used to infer source rock type across the deep water Gulf of Mexico. In areas where clastic sourcing is prominent, lower sulfur oils are predicted, whereas in areas dominated by carbonate signatures, higher sulfur oils will be present.

The Scarface thrust of the western Madison Range, Montana, is a 17° west-dipping Late Cretaceous thrust that places Archean gneisses over a complexly folded panel of Phanerozoic sedimentary rocks. The Archean-Cambrian contact on the footwall of the Scarface thrust is nearly vertical, and both bedding in the cover and foliation in the gneisses near the contact were rotated by 38° during folding. Paleozoic rocks up section in the footwall are overturned, with an axial surface that dips less than 10° west. The Scarface thrust is locally folded over lower Paleozoic rocks on the footwall. Folding was produced by post-Scarface thrust movement on a minor east-dipping splay fault that follows bedding in Devonian rocks. Of the two dominant shear fracture and fault sets in the basement (strikes and dips of N52°W, 47°NE; N20°W, 50°SW), the northeast-dipping set is parallel to foliation and reflects a strong influence of foliation on basement deformation. Intergranular fractures nucleated at the tips of biotite grains. Narrow zones of cataclasis containing shredded biotite formed along the intergranular fractures. Advanced stages of deformation were accompanied by formation of thicker zones of wavy, foliated cataclasites defined by dark seams of comminuted biotite, feldspar, and quartz. The recumbent footwall syncline is superimposed on the west limb of a large, more open syncline in Paleozoic and Mesozoic rocks. We are unable to resolve which fold formed first. Faulting sequences are also equivocal. The Scarface thrust may have been emplaced as a shallowly dipping sheet, or it may have been steeper initially and rotated during movement on the structurally lower Beaver Creek thrust.

Abstract The Upper Cambrian Bonneterre formation (400 feet) is widely mineralized, predominantly with galena. The Bonneterre, dolomite or limestone, is underlain generally by highly porous Lamotte sandstone (0–500+ feet) resting on a Precambrian basement that rises in many places into the carbonate series. Above the Bonneterre is the Davis shaly limestone (150 feet), succeeded, where not eroded, by several hundred feet of dolomites which contain extensive barite deposits with accessory galena. Isotopic studies show conclusively that all ore lead in the Paleozoic beds is anomalous, of J-type, varying considerably within deposits. In general, any vertical section of Bonneterre ore lead, as in a drill hole, is distinctly more radiogenic at the base of the formation, less so upward. Furthermore, any large mineralized area is generally more radiogenic centrally, along guiding faults or controlling basement highs (knobs), and less toward perimeters. Finally, in all clear-cut cases noted, the earlier deposited galena is least radiogenic. Note, however, that the trace galena in overlying barite deposits is notably more radiogenic than in the Bonneterre ore bodies, apparently due to association with large deep-seated faults. Traces of post-Precambrian lead in the basement are excessively radiogenic. However, minor veinlets of typically Precambrian galena also are known. Sulfur isotopes are virtually identical with sea water sulfate (21.80) in the less radiogenic lead but in more radiogenic galena are lighter (22.00). Sulfur variations also show definite stratigraphic relations to horizons of high fossil content (reef rock) in the Bonneterre. The conclusion is that radiogenic lead from the Precambrian basement mingled with normal lead in the connate fluids of the sediments to yield the anomalous hybrids. The relative proportion of the basement contribution possibly was on the order of one-third of total lead.