4 results for Pearson, Michael J.

  • Jurassic septarian concretions from NW Scotland record interdependent bacterial, physical and chemical processes of marine mudrock diagenesis

    Hendry, James P.; Pearson, Michael J.; Trewin, Nigel H.; Fallick, Anthony E. (2006-05)

    Journal article
    University of Waikato

    Septarian concretions in the Staffin Shales Formation (Kimmeridgian, Isle of Skye) allow controls on concretion rheology and septarian cracking to be investigated. Stratabound concretions consist of anhedral ferroan calcite microspar enclosing clay and minor pyrite. Intergranular volumes range from 77% to 88%, and calcite δ¹³C and δ¹⁸ O values in most concretion bodies range from −10•0‰ to −17•3‰ and +0•3‰ to −0•6‰ respectively, consistent with rapid and pervasive cementation in marine pore fluids. Septarian rupture occurred during incipient cementation, with a sediment volume reduction of up to 43%. Crack-lining brown fibrous calcite records pore fluid re-oxygenation during a depositional hiatus, followed by increasing Fe content and δ¹³C related to bacterial methanogenesis. Brown colouration results from an included gel-like polar organic fraction that probably represents bacterially degraded biomass. A new hypothesis for concretion growth and septarian cracking argues that quasi-rigid 'proto-concretions' formed via binding of flocculated clays by bacterial extracellular polysaccharide substances (EPS). This provided rheological and chemical conditions for tensional failure, subcritical crack growth, volume contraction, calcite nucleation, and incorporation of degraded products into crack-lining cements. Bacterial decay of EPS and syneresis of host muds provided internal stresses to initiate rupture at shallow burial. Development of septarian (shrinkage) cracks in muds is envisaged to require pervasive in situ bacterial colonization, and to depend on rates of carbonate precipitation versus EPS degradation and syneresis. Subsequent modification of septarian concretions included envelopment by siderite and calcite microspar, hydraulic fracturing associated with Cretaceous shallow burial or Palaeogene uplift; and cementation by strongly ferroan, yellow sparry calcite that records meteoric water invasion of the host mudrocks. An abundance of fatty acids in these spars indicates aqueous transport of organic breakdown products, and δ¹³C data suggest a predominantly methanogenic bicarbonate source. However, the wide δ¹⁸O range for petrographically identical cement (−1•3‰ to −15•6‰) is difficult to explain.

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  • Tubular concretions in New Zealand petroliferous basins: Lipid biomarker evidence for mineralisation around proposed miocene hydrocarbon seep conduits

    Pearson, Michael J.; Grosjean, Emmanuelle; Nelson, Campbell S.; Nyman, Stephanie Leigh; Logan, Graham A. (2010)

    Journal article
    University of Waikato

    Trapped organic compounds (lipids) have been analysed in tubular carbonate concretions and their host sediments in Miocene deep water mudrocks from coastal outcrops in East Coast Basin and Taranaki Basin of North Island, New Zealand. The concretions, including calcitic, dolomitic and mixed mineralogies, have varied morphologies in many cases suggestive of conduits or pipes that channelled the escape of subsurface fluids and/or hydrocarbon gases. The extracted lipids include water column and/or diagenetically-derived alkanes, fatty acids and alcohols as well as specific marker compounds (including archaeal pentamethylicosane (PMI) and archaeol) associated with subsurface anaerobic oxidation of upwardly seeping methane gas (AOM). Strong carbon-13 isotopic depletions (δ13C –75 to –120‰) measured for PMI, archaeol and other AOM-specific marker compounds on three concretion samples support involvement of AOM in generating bicarbonate-rich fluid that was at least partly responsible for cementing the pipe-like concretions and central conduits. Other morphological types appear not to be AOM-related. Sterane and n-alkane parameters indicate low thermal maturity of the extracted organic matter. The molecular and compound specific isotopic organic geochemical evidence that some tubular concretions functioned as methane conduits thus supports an assertion that the tubular concretions represent 'fossilised' parts of the subsurface plumbing of biogenic or thermogenic hydrocarbon-fed cold seep systems.

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  • Organic chemical signatures of New Zealand carbonate concretions and calcite fracture fills as potential fluid migration indicators

    Pearson, Michael J.; Nelson, Campbell S. (2006)

    Journal article
    University of Waikato

    Macroscopic calcite crystals are common in sedimenta¬ry strata, occurring both as tectonic veins and also filling one or more generations of septarian rupture or later brittle fractures in calcareous concretions. Traces of hydrocarbons are frequently present in calcite crystals, especially near active petroleum systems, and are routinely the object of fluid inclusion studies linking source and migration pathway. Such calcites are shown here also to contain fatty acids in widely varying amounts ranging from 0.2 to more than 5 μg/g. Vein calcites examined are typically near the lower figure, close to analytical blank levels, and this is also true of some concretionary fracture fill calcites, notably those from the Palaeocene Moeraki ‘boulders’. Other concretionary fracture fill calcites (Jurassic, Scotland; Eocene, Waikato Coal Measures and associated marine strata) have much higher fatty acid contents, especially those filling later brittle style fractures. Although usually less abundant than the fatty acids in the concretions themselves, they lack the long chain n-acids derived from terrestrial vegetation and are commonly dominated by dioic acids. Exceptionally, in the calcitic septarian fill of a sideritic Coal Measures concretion, their abundance far exceeds that of concretion body fatty acids. They appear to be fluid transported, probably in aqueous solution, and have molecular signatures potentially distinctive of maturing organic matter sources from which the fluids derived.

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  • Tubular carbonate concretions as hydrocarbon migration pathways? Examples from North Island, New Zealand

    Nyman, Stephanie Leigh; Nelson, Campbell S.; Campbell, Kathleen A.; Schellenberg, Francesca; Pearson, Michael J.; Kamp, Peter J.J.; Browne, Gregory H.; King, Peter R. (2006)

    Conference item
    University of Waikato

    Cold seep carbonate deposits are associated with the development on the sea floor of distinctive chemosyn¬thetic animal communities and carbonate minerali¬sation as a consequence of microbially mediated anaerobic oxidation of methane. Several possible sources of the methane exist, identifiable from the carbon isotope values of the carbonate precipitates. In the modern, seep carbonates can occur on the sea floor above petroleum reservoirs where an important origin can be from ascending thermogenic hydrocar¬bons. The character of geological structures marking the ascent pathways from deep in the subsurface to shallow subsurface levels are poorly understood, but one such structure resulting from focused fluid flow may be tubular carbonate concretions. Several mudrock-dominated Cenozoic (especially Miocene) sedimentary formations in the North Island of New Zealand include carbonate concretions having a wide range of tubular morphologies. The concretions are typically oriented at high angles to bedding, and often have a central conduit that is either empty or filled with late stage cements. Stable isotope analyses (δ13C, δ18O) suggest that the carbonate cements in the concretions precipitated mainly from ascending methane, likely sourced from a mixture of deep thermogenic and shallow biogenic sources. A clear link between the tubular concretions and overlying paleo-sea floor seep-carbonate deposits exists at some sites. We suggest that the tubular carbonate concretions mark the subsurface plumbing network of cold seep systems. When exposed and accessible in outcrop, they afford an opportunity to investigate the geochemical evolution of cold seeps, and possibly also the nature of linkages between subsurface and surface portions of such a system. Seep field development has implications for the characterisation of fluid flow in sedimentary basins, for the global carbon cycle, for exerting a biogeochemical influence on the development of marine communities, and for the evaluation of future hydrocarbon resources, recovery, and drilling and production hazards. These matters remain to be fully assessed within a petroleum systems framework for New Zealand’s Cenozoic sedimentary basins.

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