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BROWNE, GREG H., Institute of Geological & Nuclear Sciences, Lower Hutt, New Zealand; and ROGER M. SLATT, Colorado School of Mines, Golden, CO, USA

Abstract: An Integrated Outcrop and Behind Outcrop Study of Slope San (Channel-levee) Lithofacies, Taranaki, New Zealand, and Their Implications to Thin-Bedded Reservoirs

Lithofacies from the Late Miocene Mt Messenger Formation in Taranaki are a useful analogue of thin-bedded, deep-water reservoirs.An integrated outcrop and subsurface study undertaken in coastal outcrop and behind outcrop locations is described in this study. Outcrop work consisted of detailed lithofacies description, petrophysical determinations, and mapping of stratal geometries. Subsurface work included drilling two fully cored stratigraphic holes spaced 150 m apart, and about 100 m behind the outcrop. The holes were drilled to 120 m and 70 m depths, respectively, and logged using FMI* and Platform Express* (*TM Schlumberger Wireline & Testing Co.) logging suites. In addition, a high resolution seismic reflection line was acquired along the beach immediately adjacent to the coastal outcrop. From these data, we were able to tie several key stratigraphic horizons from the drillholes into the outcrop, and to recognise the equivalent strata in the seismic reflection line.

Typically the formation consists of well sorted, very fine-grained to fine-grained, thin-bedded sandstone (bed thickness generally less than 30 cm), dominated by horizontally bedded, climbing ripple laminated sandstone, and massive siltstone. Sandstone porosity between 25-35% and permeability averaging 200 mD (up to 750 mD) have been recorded from both the outcrop and the cores. Bioturbation is abundant in both sandstone and siltstone lithologies, though the type of burrows occurring in each lithology is quite distinct. Scolicia burrows are abundant within climbing ripple laminated sandstone intervals, and indicate very rapid sedimentation rates.Anonichnus and Zoophycos burrows dominate siltstone rocks. Many beds display well developed Bouma cycles, interpreted as turbidite deposits, though there are minor debris flow conglomerates and slump beds present. Foraminifera indicate middle and upper bathyal depositional depths during the Late Miocene.

From the above data sets we interpret the outcrop section to represent a series of stacked slope fans (channel-levee) deposits comparable to Mutti type III fans. Three major lithofacies are recognised. Channel fill lithologies consist of thin-bedded siltstone and alternating sandstone and siltstone that display an upward decrease in dip magnitude and variable dip orientation on dipmeter logs (Fig. 1). Channel bases are sharp and highly erosive, and may be filled initially with siltstone, passing upward into alternating beds, or may show thinning upward, alternating sandstone and siltstone units. Thinner, vertically stacked, interleaved packages of thin-beds with complex upward-decreasing dipmeter patterns are interpreted as proximal channels and overbank deposits (Fig. 2). In outcrop, channel bases may be lined with a conglomerate lag, centimetres to several metres thick, and are filled by alternating sandstone and siltstone strata, or by siltstone. Laterally extensive, sheet-like alternating sandstone and siltstone lithologies with low-angle dips and little erosional truncation of beds are interpreted as distal fan and overbank deposits. They typically have a complex upward-decreasing dipmeter pattern. Gamma ray logs typically show little variation throughout the different depositional elements, presumably because of the muddy or fine-grained nature of the sediment.

In seismic, reflectors are both relatively continuous and parallel, or show erosional truncation. Laterally extensive interbedded lithologies with low-angle dips and a uniform dipmeter.pattern are interpreted as distal fan and overbank deposits. Packages of strata several metres thick, with opposing attitude are evident in the seismic line shot along the beach. Packages are marked by distinct boundaries, with overlying beds onlapping these surfaces. We interpret these to be distinct slope fan or levee packages, similar to those observed in outcrop from both the proximal and distal channel and overbank strata.

Outcrop and core data display abundant cut and fill structures. Erosional scours limit bed continuity especially in proximal channel and overbank settings. Scours are often lined with a thin mud drape, and this will considerably limit the flow of hydrocarbons through similar reservoir facies.

We believe the outcrop and subsurface data presented in this study are applicable to Mt. Messenger Formation strata in the producing Ngatoro and Kaimiro fields, 30 km to the south in central Taranaki. In addition, we believe many similar features occur in other fields around the world, particularly in the Gulf of Mexico. For example, in the Ram/Powell and Mahogeny Fields, Gulf of Mexico, the "L" sands include large channel and levee complex systems, which have scour phenomina, channel filling facies, and thin-bedded alternating sandstone and siltstone in more distal levee flanks.

AAPG Search and Discovery Article #[email protected] International Conference and Exhibition, Birmingham, England