A study investigated material flux and porosity changes during sediment diagenesis in ten reservoir sandstones from oilfields worldwide. The results suggest that the flux of silica cannot be modeled ...satisfactorily using currently accepted values of permeability, silica solubility and flow rates, posing a challenge for existing models of fluid flow and ore emplacement.
Several wells in the Amethyst Gas field of the North Sea's Southern Basin are poor producers and have been since they were drilled. The lack of gas flow from these wells is due to pervasive ...cementation of the Rotliegend sandstone reservoir by either anhydrite and/or barite. Both minerals precipitated late in the diagenetic history of the sandstones. Such cements from up to 20% of the total rock.
Isotopic and geochemical evidence indicate that the source of the elements for these sulphate cements was outside the Rotliegend sandstone. The sulphur and oxygen isotope data for the anhydrite and barite are unlike those which could have precipitated in Lower Permian times from an evaporating marine basin. Nor are they like any sulphur and oxygen isotope ratios that might be derived from a provenance area for the Rotliegend Sandstone. However the sulphur and oxygen data are identical to anhydrite in the overlying Upper Permian, Zechstein carbonate and evaporate sequences.
The Rotliegend sandstones did not contain much barium at deposition. However, unlike the sulphate, the Zechstein sequences are a highly unlikely source for barium, given the highly insoluble nature of barite. A more probable source for barium is the underlying Carboniferous Coal Measures.
Precipitation of barite and anhydrite occurred during basin inversion. Specifically the cements are most abundant in gas wells that lie close to inverted normal faults that cut both the Permian and Carboniferous intervals. Both the timing and distribution of these cements is taken to indicate that faulting allowed, or indeed promoted, mixing of sulphate-rich and barium-rich formation waters derived from the Zechstein and Carboniferous, respectively.
In New Year 1938, the Edale No. 1 oilwell was drilled in the English Pennines. The target for the well was folded Lower Carboniferous limestone. This target was believed to be analogous both for trap ...and reservoir, to the giant oil accumulations of Iran (then Persia). The well failed. Poor gas shows were recorded at a depth of 126feet. This gas flowed at a rate of 4000ft
3 day
−1. There were also minor indications of brown and green oil but no commercial production. Trap and seal were proven. Local seepage also proved the existence of a working source. Reservoirwas however missing. The well was drilled outwith the play fairway. Instead of penetrating massive, porous shelf limestones the well encountered thin shaley limestones, today interpreted as deep water, gravity flow deposits.
This paper recounts the geological work done before and during drilling of the well. The data, in the form of a prospect evaluation memorandum, are from the BP Exploration archive.
Aeolian, Rotliegend sandstones from the Village Fields area of the southern North Sea were cemented by illite and kaolinite during the latter part of the Jurassic coincident with a phase of active ...rifting in the basin. The components for the formation of the clays could not have been supplied from the original detritus, which was rich in quartz and poor in feldspar and lithic fragments. Instead, large quantities of aluminium, potassium and silicon must have been supplied to the sandstone from elsewhere. The source for the potassium is likely to have been either the laterally equivalent Silverpit Claystone Formation or the overlying Zechstein evaporite sequences which were plumbed into the Rotliegend during the late Jurassic rifting. The source of silica and alumina has not been uniquely identified.
Norman Oxtoby & Jon Gluyas write: Wilkinson & Haszeldine (1996) have added to the lively debate on the mobility of elements during the diagenesis of silicate minerals in sandstones. They concluded ...from a petrographic study of the Fulmar Formation (Jurassic, North Sea) that large quantities of aluminium (Al) have been lost from the sandstone during the course of its burial history. While we have no objection to the notion that large volumes of a variety of elements can be moved in or out of sandstones (cf. Evans 1990; Gluyas & Coleman 1992*), we believe that the evidence presented by Wilkinson & Haszeldine is ambiguous. We develop equally plausible interpretations of their data which lead to conclusions of aluminium content stasis or even import!
According to our reading of their paper, Wilkinson & Haszeldine (1996) present the argument that, if the initial composition of the Fulmar Formation was >35% feldspar, then a mass balance of Al-bearing minerals quantified from petrographic studies shows that c. 1500 μmol cm–3 Al has been exported from the most deeply buried sandstones in the studied area. We show below that we need to be more certain about the validity of the 'if condition, before accepting many of the conclusions of this paper, including major Al loss. We do this by commenting on the statements made in the conclusions, shown in italics in the following.
(1) The Upper Jurassic Fulmar Formation sandstones show a steady decline of K-feldspar abundance with increased burial. This is partially correct
The SEED (Student Enterprise Experience in Dunedin) programme was developed as a four-week, intensive entrepreneurial ‘boot camp’ to provide a small group of participants with a highly experiential ...business course. Using pre-course and post-course surveys, the authors measured the entrepreneurial ability, knowledge and intentions of the sixteen SEED participants to determine whether the programme was successful as an educational format. The results showed that perceived entrepreneurial ability and knowledge significantly improved, while intent increased but not significantly. Almost all participants intended to start a business both before and after the course, while twice as many (six) indicated that they would undertake further formal study in entrepreneurship after the course compared to before. The data suggest that SEED is filling a niche in entrepreneurial development and support for budding entrepreneurs, with the course also facilitating local business development.
