Characterization of mixed siliciclastic/carbonate systems within the Mulichinco Fm (Valanginian) in Cañadón Amarillo field, northern Neuquén Basin (Argentina). An integrated approach to build more robust exploration and production models

Abstract

An integrated study was carried out in Mulichinco Formation (Cañadon Amarillo Field, CAM) aiming at characterizing the reservoir by building a predictive model both at exploration and production scale. This study was based on the integration of high resolution sequence stratigraphy, log analysis and seismic attribute interpretation. The Mulichinco Formation developed after a second-order, tectonically-influenced Early Valanginian sea-level fall. In CAM field it consists of shallow-marine siliciclastics and carbonates deposited within an epeiric seaway. Mulichinco strata (140-110 m thick) can be divided into three stratigraphic intervals (Lower, Middle and Upper Members) characterized by different lithologies and seismic geometries. The Lower Mulichinco Member (35-20 m thick, thinning to East) comprises two to three, poorly-defined coarsening-upward successions (8 to 15 m each) with bioturbated heterolithics (or less commonly mudstones) at the base, grading upward to sandstones with hummocky cross-stratification (HCS) and ripple cross-lamination. Commonly, cross-stratified calcarenites and/or oolitic grainstones cap the successions. These cycles are interpreted as regressive parasequences, representing siliciclastic-dominated proximal offshore settings to mixed shoreface conditions. An internal reflector matching a flooding surface and pinching out toward the East defines a proximal (East) to distal (West) facies trend for this stage. Tide-influenced carbonate shoals and mixed/siliciclastic bars replaced laterally in the shallower portion of the ramp. Within the Lower Mulichinco, reservoir potential is restricted to some discontinuos sandstone bodies (< 2m thick) with poor petrophysical properties interpreted as isolated mixed/siliciclastic bars. The Middle Mulichinco Member (25-35 m thick) consists mainly of micrite-rich, bioclastic floatstones and boundstones dominated by epibenthic macrofauna (oysters and serpulids), interbedded with less frequent micrite-poor floatstones with endobenthic fauna (mainly bivalves). The pure-carbonate interval is replaced towards the top by very fine-grained bioturbated calcarenites. Oyster-rich carbonate accumulations are interpreted as low-relief mounds within a gently dipping carbonate ramp, whose regional extension largely exceeds the study area. The top of the Middle Mulichinco is interpreted as a regionally significant maximum flooding zone. Moderate reservoir conditions are expected in this section within some micrite-poor floatstones having intramoldic and intermoldic porosity; these beds are quite tabular, thin and continuous (< 2 m thick). The Upper Mulichinco Member (60-75 m thick) shows a well-developed vertical cyclicity. Cycles thicken upwards (4 to 22m each), have a sharp base and comprise a basal carbonate-rich hemicycle overlain by a siliciclastic-dominated hemicycle. Carbonate hemicycles consist of micrite-rich, bioclastic wackestones and floatstones, locally associated with some cross-stratified grainy packstones and grainstones. Skeletal-rich facies are gradually replaced by siliclastic dark, laminated shales, passing upward to bioturbated sandy mudstones and muddy sandstones, an eventually into clean, very fine to fine-grained sandstones typically with HCS. Cycles are interpreted as genetic units reflecting high-frequency, low-amplitude relative sea-level oscillations, in which transgressive conditions inhibited terrigenous input and allowed for the development of a carbonate-dominated system, whereas normal regressive, siliciclastic-dominated conditions are marked by the transition from offshore shales to middle shoreface sandstones. Seismic geometries of the Upper Mulichinco cycles show E/NE prograding, low-relief clinoforms revealing a complex spatial distribution of facies with good reservoir potential, represented by middle shoreface sandstones and grainy packstones/grainstones in the uppermost part of the cycles.