Reservoir Aspects Of Submarine Fan Facies: Example Of The Marnoso Arenacea Formation, Northern Apennines, Italy

Abstract

ABSTRACT The Marnoso Arenacea Formation was deposited in a deep marine trough as a cyclic sequence of prograding submarine, fans that merge down dip into basin plain deposits. Rock outcrops and sandstone samples of the formation were studied to determine which fan deposits have the best hydrocarbon reservoir potential. Sandstones from the basin plain, outer fan and middle fan facies are thin (< 2 m thick), isolated by impermeable calcareous shale beds, and are completely cemented by calcite. In contrast, channel-fill sandstones of the inner fan facies form stacked sequences up to 100 m thick and 1 km wide that have good vertical permeability and porosities from 20 to 32%. All sandstones of the formation were cemented by calcite before much compaction took place, but the channel-fill sandstones of the inner fan facies were almost totally decemented during a subsequent episode of migration of acid formation waters. Thus, the submarine fan facies that had the best reservoir quality at the time of deposition still has the best reservoir quality after many diagenetic events. Other ancient submarine fan deposits may have had a similar history. INTRODUCTION In offshore areas production of hydrocarbons are chiefly from sandstones that were deposited in deltaic, shoreline, and deep-water environments. The geometry and reservoir potential of deltaic and shallow marine sandstones are fairly well known from work over the past 20 years, but little has been published about the reservoir aspects of sandstones deposited in deep water. Stratigraphic and sedimentologic studies of modern and ancient deep-water deposits from various parts of the world show that sandstones were deposited chiefly in submarine fans and adjacent basin plain environments (Fig. 1). The Marnoso Arenacea Formation is an example of a sequence of prograding submarine fans that interfinger with basin plain deposits that were deposited in a deep marine trough. The formation, several thousand meters thick and of Miocene age, was subsequently thrust faulted onto the Italian peninsula where it is now exposed in canyons and road cuts in the northern Apennines. Sandstone samples of the different deep-water facies of the Marnoso Arenacea were studied to determine which facies have the best reservoir quality and hence the greatest hydrocarbon reservoir potential. Although the formation has not produced hydrocarbons in the Apennines, the results of this study should be useful in predicting the reservoir characteristics of similar deepwater sandstones in other regions. PREVIOUS WORK Italian workersl, 2 have identified the deposits of five different submarine fan environments from a study of the Marnoso Arenacea Formation in outcrop. The facies were identified by comparison with studies of modern deep-sea submarine fans. The deposits are from the basin plain, outer fan, middle fan, inner fan, and slope environments; the stratigraphic aspects of these rocks are shown schematically in Figure 2. Because submarine fans (and the slope up-dip from them) generally prograde basinward it is common to find these various facies in a vertical succession with basin plain deposits at the base and slope deposits at the top.