Reservoir-Facies Distribution Within Stacked Channel Sands of Falling-Stage/Lowstand Systems on Continental Margins with High Sediment Supply and a Low-Gradient Continental Shelf: Results From Analyses of a Modern Analog—Yellow Sea
Authors: Moss C. C. , Bartek Louis R. , Pearce P. 
Published: April 2012

Stacked channel sands from braided fluvial deposits have attracted interest as excellent hydrocarbon reservoirs because of their high porosity, high permeability, and large volume due to great thickness and lateral extent. Strata of this nature are commonly found in epicontinental-seaway depositional successions. Strata of the Yellow Sea (YS) continental margin compose an excellent modern analog of epicontinental-sea and foreland-basin depositional settings because of the YS’s very high sediment supply, shallow depth, and gentle gradient from the shelf edge to basin. Today, these conditions are rarely found on modern continental margins, but they were quite significant in ancient continental-margin stratigraphic successions. Therefore, a grid of approximately 5,024 km of high-resolution (&lt;1 m) seismic-reflection and chirp-sonar data and cores were collected from the YS to begin to develop models of the spatial variability of reservoir facies that evolve in this type of system. These data have been processed, filtered at 300 to 2,000 Hz, and interpreted in a sequence stratigraphic framework. Seismic facies were objectively identified on the basis of reflection orientation, frequency, amplitude, and lateral continuity. These seismic facies are assumed to be related to the mode of sediment deposition and are further interpreted to be indicative of deposition of specific lithofacies within particular systems tracts.

Pleistocene falling-stage/lowstand deposits identified on high-resolution seismic-reflection data and chirp-sonar data from the YS continental shelf (and epicontinental sea) consist of seismic facies with reflections that are chaotic or steeply inclined. These facies are located above an erosional unconformity (sequence boundary) that truncates underlying and laterally adjacent reflections. The great lateral extent of these falling-stage/lowstand deposits (nearly 400 km in the southern YS and 100 km in the northern YS) suggests deposition from a rapidly avulsing, braided fluvial depositional environment. The falling-stage facies are locally stacked on other channel sands and separated by thin transgressive systems (silty fine sand), which define prominent transgressive surfaces. Thickness of the channel-sand units is commonly 25 m. These channel sands are capped by organic-rich muds (as thick as ~2 to 10 m). Stratified silty mud deposits occur adjacent to the channel-sand packages. An early assessment of the horizontal and vertical distribution of seismic facies suggests that falling-stage braided fluvial deposits are prevalent in the southern end of the YS (reservoir rock that is relatively homogeneous, with very high lateral continuity, and a volume of ~1,400 km3). Highstand deposits (identified by downlapping, nearly flat lying, laterally continuous parallel reflections with moderate to high frequency) are more widespread both vertically and horizontally in the north, behind the Shantung-Laoyehling Massif. The Fukien-Reinan Massif is also apparent in the seismic records. Maps of the distribution of these falling-stage/lowstand deposits indicate that an important controlling influence on the distribution of these deposits is the paleogeographic location of preceding lowstand channels and the locations of the crystalline Shantung-Laoyehling Massif in the north and the crystalline Fukien-Reinan Massif in the south. The crystalline rocks serve as barriers to fluvial flow when they are exposed during sea-level lowstands, and they protect certain areas from fluvial reworking and erosion during lowstands. In these protected areas, settling of silt from suspension during sea-level highstands is the dominant depositional mechanism.

The high lateral continuity (along both strike and dip) of the falling-stage/lowstand reservoir facies of depositional systems such that of the YS (which ranges from 100 km updip to nearly 1,000 km downdip) is significantly different from that of many Gulf of Mexico systems (which commonly have a maximum lateral continuity of 50 km). In addition to the high continuity of reservoir facies, systems such as the YS also have a seal of fine-grained highstand deposits that lie over the fluvial facies. These highstand deposits are also laterally continuous (over 100 to 1,000 km) and contain a significant quantity of organic matter. They, therefore, also may serve as source rocks.