MTCs of the Brazos-Trinity Slope System; Thoughts on the Sequence Stratigraphy of MTCs and Their Possible Roles in Shaping Hydrocarbon Traps

Abstract

Mass Transport Complexes (MTCs) are significant constituents of the fill of the basins of the Brazos-Trinity Slope System. These MTCs are composite stratigraphic bodies consisting of resedimented materials associated with slumps, slides, debris flows, but also subordinate turbidites, and hemipelagites. Although their composition is variable, they tend to be mud-rich. The MTCs exhibit a variety of external geometries. They commonly have erosive bases characterized by “boxy”, grooved scours and mounded tops. Commonly, they are internally chaotic, but sometimes show crude stratification or fabrics related to patterns of movement and/ or deformation. Because of increased awareness of the abundance of MTCs within deep marine settings and their association with other well known architectural elements, recent attempts have been made to incorporate the occurrence of MTCs into predictive sequence stratigraphic models. The models emphasize the role of relative sea level cycles in the generation of MTCs and relate this to formative processes and positions within an idealized depositional sequence. However, while some Brazos-Trinity MTCs may conform to these concepts, others do not. As such, utility of MTCs for developing sequence stratigraphic frameworks for the Brazos-Trinity system is dubious. Based on our understanding of the Brazos-Trinity Slope System it is proposed that MTCs play a role in shaping hyrdrocarbon traps in analogous subsurface systems. Although some portions of these MTCs contain sufficient porous and permeable sediment to constitute hydrocarbon reservoirs, most represent seal and “waste rock” units. They can form basal, vertical and lateral seals to reservoir complexes, as well as intra-reservoir seals and baffles. Because of their erosive nature, the emplacement of MTC's has often resulted in localized or widespread termination of reservoir strata. MTC's with clay-rich lithologies form low permeability, high capillary entry pressure layers overlying these erosional surfaces. Thus, the geometry and transmissibility of contacts between MTCs and reservoir-bearing strata can play an important role in defining the stratigraphic component of hyrdrocarbon traps and represent baffles or barriers to fluid flow during production.