Anatomy, stratigraphic distribution and reservoir implications of channel units in a fluvial-tidal interaction system: the Mulichinco Formation in central Neuquén Basin, Argentina

VEIGA, G.D.; SCHWARZ, E.

Manchester

Congreso; 30th Meeting of Sedimentology; 2013

International Association of Sedimentologists

In fluvial-tidal interaction systems, the degree of tidal influence in distal fluvial settings has a great impact in the nature of channel deposits. Changes in current strength or even slack periods promoted by tidal influence downstream fluvial channels may promote changes in grain size, internal architecture and internal heterogeneity in resultant channel deposits. This may eventually condition their behaviour as hydrocarbon reservoirs in the subsurface. Moreover, high-frequency changes in relative sea level in distal fluvial/inner estuarine systems will produce facies belt shifts that can impact in the amount of tidal influence in one location, adding complexity to the vertical distribution of channel deposits. The Valanginian Mulichinco Formation is characterised in south/central Neuquén basin by the transition from relatively proximal, coarse-grained, fluvial deposits at the base to estuarine facies at the top, in a clear transgressive trend. The lower 120 m of this unit were studied in outcrop in order to define the main characteristics, the degree of tidal influence and the stratigraphic distribution of channel deposits. Three types of large-scale channel deposits were identified in the lower Mulichinco Formation taking into account their internal architecture, facies distribution, orientation of internal discontinuities and grain size. These channel types are thought to represent fluvial deposits with different degree of tidal influence in a distal fluvial/upper estuarine setting. The internal architecture and grain sizes change considerably between different channel types, affecting the potential performance as hydrocarbon reservoirs of these units. Some channels (Type 1) show relatively coarse grain size (up to fine conglomerate) and a complex internal architecture that show no evidence of tidal influence. Other channels (Types 2 and 3) show a more simple internal anatomy with large-scale inclined surfaces that indicate the predominant lateral migration of these channels. These also show a finer grain size (fine- to medium-grained sand) and even the alternation of sandy and heterolithic deposits in the large-scale inclined strata. Type 2 and 3 channels are thought to represent a higher degree of current variability probably associated with current modulation promoted by tides. However, the external geometry and dimensions of large-scale channels do not show significant changes in relation to the degree of tidal influence. All these channel deposits are thin to very thin units (between 1 and 4.6 m) that can amalgamate to constitute coarse-grained units up to 7 m thick. Lateral extension ranges between less than 50 and over 450 m with a thickness/lateral extension ratio between 50 and 257 (narrow to wide sheets). Vertical changes in channel type are also linked to changes in vertical amalgamation of channel deposits and the preservation of fine-grained facies. These changes are believed to reflect high-frequency changes in the ratio between accommodation creation and sediment supply which, in turn, might have controlled the position of facies belts, and therefore the type of channels accumulated, together with the rate of coastal plain/inner estuary aggradation. Significant vertical changes in channel properties might not only be important in characterising the reservoir potential of these successions but also for understanding/predicting high-frequency changes in stacking and type of marine deposits accumulated in coeval distal sequences as the stratigraphic distribution of channel types might shed light into the evolution of accommodation creation and sediment delivery into the marine realm.