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SYNOPSISOutcrops of Middle-Upper Jurassic rocks in northern Scotland are restricted to two sedimentary basins. In the east exposures at Brora (Sutherland) and Balintore (Ross-shire) are part of the Moray Firth basin.' In the west major exposures at Staffin and in Strathaird (Isle of Skye) belong to the Minch basin. A formal lithostratigraphy is erected with five formations and twenty-six members, which have been correlated with the standard ammonite zone. Each basin presents a pair of sections, with thick sandy onshore sequences at Brora and Strathaird and condensed argillaceous offshore sequences at Balintore and Staffin. The two onshore/offshore localities show marked facies similarities, but are affected by different transgressions in the Oxfordian.A new species ofLongaeviceras is described. It is the first authenticated record of this genus from the Oxfordian.

Pyrolytic analyses suggest that Cretaceous coaly

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International audienceRock—Eval HI values for coals vary with rank and do not give a direct measurement of oil potential. However, oils from coals are characteristically paraffinic and can be considered to derive from a polymethylene (PM) component, so the PM content should provide an estimate of the paraffinic oil potential. A trend apparently representing lignin evolution has been identified on the Van Krevelen diagram which permits the relative proportions of carbon in lignin and PM to be determined for coals that approximate a mixture of these two components, such as the members of the New Zealand (NZ) Coal Band. On the basis of this compositional model, HI values can be calibrated to provide an alternative estimate of the paraffinic oil potential. A maximum in HI is generally reached in coals near the onset of oil generation, at Rank(S) 12 (R. ca. 0.7%), from which it is suggested that the PM contri¬bution can be obtained using the formula HIpm = 1.15HIma„-172 for the suite of NZ coals examined. The onset of oil expulsion can be identified from a variety of geochemical measurements, and occurs in the Rank(S) range ca. 12.0 — 14.5 (Ro ca. 0.7-1.1%) for coals with paraffinic oil potentials exceeding ca. 40 mg HC/g TOC. Data from Taranaki Basin coals correlate well with the theoretical relationship between BI/HIPM and HIpm, using bitumen index (BI = S 1/TOC) values of 10 mg HC/g TOC at the start of oil generation (i.e. bitumen inherited from diagenesis) and 40 mg HC/g TOC at the onset of oil expulsion, suggesting the HIpm model is reasonably accurate for members of the NZ Coal Band. Kinetic modelling of paraffinic oil generation from vitrinite-rich coals may be best approximated by consideration of PM degradation alone

The Late Cretaceous Rakopi Formation (Pakawau Group) represents one of the most important petroleum source rock units and a potential reservoir unit in the highly prospective Taranaki Basin. This paper presents a predominantly outcrop-based study of the sedimentology, petrography, stratigraphy, and depositional environment of the Rakopi Formation in the Paturau River and Pakawau areas of northwest Nelson, southern Taranaki Basin, together with some preliminary insights into the stratigraphie architecture of the Pakawau Group on a more basin-wide scale.The Rakopi Formation is interpreted here as a terrestrial deposit, representing sedimentation in fluvial channels and their associated overbank and levee environments. However, the presence of dinoflagellates, glauconite, and elevated coal seam sulfur contents is evidence for periodic marine influence during deposition. This could be explained by a low-gradient coastal plain paleogeography, crossed by a series of rivers and their associated floodplain deposits, episodically inundated by marine incursions during successive transgressions. A modern analogue setting from the present-day Hauraki Graben, North Island, New Zealand, indicates that marine influence within coastal plain systems can extend several tens of kilometres inland. Given such a physiography, relatively small increases in relative sea level could potentially move the shoreline several kilometres or tens of kilometres farther inland, sufficient to introduce the type of marine influence on sedimentation that we suggest for the Rakopi Formation.The results from this study suggest a greater marine influence within the Rakopi Formation, northward into the greater Taranaki Basin, than has previously been recognised. This raises the possibility of both different reservoir facies as well as potentially a greater proportion of marine mudstones, which would have implications for both reservoir and trapping of hydrocarbons. In addition, marine-influenced coaly rocks within the Rakopi Formation are expected to have greater petroleum generative potentials and to be more oil-prone than their fully non-marine counterparts.