Depositional systems and sequence stratigraphy of the M1 sandstone in Tarapoa, Ecuador
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Campanian M1 Sandstone is one of the major prospective sandstone units in the Tarapoa field in Oriente Basin, Ecuador. The M1 Sandstone is always markedly sharp based, averages 25 m in thickness, shows upward increasing marine bioturbation and generally fines upward from coarse to very fine grained sandstone. In cores, the sandstones at base are amalgamated coarse to fine grained with prominent cross stratification (dm thick), sometimes clearly bi-directional and contains mud drapes. These suggest strong tidal or fluvial-tidal currents in estuary channels or delta distributary channels. The finer grained intervals in the middle are brackish-water intensely bioturbated and dominated by mud drapes, wavy and flaser bedding suggestive of intertidal flats. Associated overlying coals and coaly shales suggest supratidal conditions. The sandstones at top are cross stratified and contain mud drapes. These again suggest strong tidal or fluvial-tidal currents in estuary channels or delta distributary channels. The stacking pattern of facies in M1 Sandstone reveals the evolution of the M1 depositional system, as well as the sequence stratigraphy of M1 sandstone. The evolution includes four stages of deposition which indicates an initial sea level rise, a subsequent sea level fall, and another sea level rise. Lateral sand-mud heterogeneity exists in the study area, forming “shale barriers”, i.e. elongate shale-rich zones that are lateral barriers to hydrocarbon migration. They are interpreted to be abandoned tidal channels filled with muddy tidal flat deposits during the sea level fall. An alternative hypothesis was established to explain the stacking pattern of facies in M1 Sandstone. A tide-dominated delta with poor fluvial input experienced intense tidal erosion and produced a sharp base at the base of M1 Sandstone. Then subtidal sand bars, intertidal flats, and supratidal sediments were deposited in sequence during a continuous regression. The core and well logs in an extension of the study area in the northwest is interpreted as more distal open shelf deposits, beyond the mouth of the Tarapoa estuary system, where transgressive tidal shelf ridges were coeval with the Tarapoa estuary system. This interpretation allows us to predict the environment between the two areas as a transition zone between tide-dominated estuary and open shelf.