Quasi-Continuous Accumulation: A New Model for Large Tight Oil Field Formation

Zhao, Jingzhou; Bai, Yubin; Cao, Qing

Tight oil is defined as oil accumulation in low-permeability non-source rock reservoirs that can produce economic flow rate only through massive fracturing or other special measures, and approximately its permeability is less than 2 md and porosity less than 12%. Tight oil is widely accepted as a continuous-type accumulation of unconventional hydrocarbons. Based on the studies of the typical tight oil fields in China, we found that large tight oil fields and part of the near-tight oil fields are neither the discontinuous conventional accumulation nor the continuous unconventional accumulation, but instead a transitional type that we refer to as quasi-continuous accumulation. The quasi-continuous hydrocarbon accumulation can be defined as accumulations occurred extensively in tight reservoirs with neither defined boundaries nor water legs and controlled by unconventional trapping mechanisms. Studies indicate that quasi-continuous tight oil accumulation is characterized by that (1) oil distribution is extensive geographically in quasi-continuous form without a defined boundary; (2) the hydrocarbon generation covers vast area and the charging is highly intensive; (3) the reservoir property is inferior and highly heterogeneous; (4) traps are unconventional that exist in a transition form between conventional shaped traps such as the structural and stratigraphic traps and the shapeless trap or no trap; (5) oil and water occurrence is complicated and no obvious edge / bottom water existed; (6) the formation pressure system is complex and underpressure is predominated; (7) the migration and accumulation is short-distanced and driven principally by non-buoyant forces; (8) oil accumulation and distribution are rarely controlled by local structures, but primarily by source and reservoir conditions; (9) the preservation condition is excellent and alteration is unobvious; and (10) the resource potential is significant. The quasi-continuous accumulation is distinguished from typical continuous-type accumulation such as coalbed methane and shale oil and gas in that quasi-continuous accumulation is occurred outside the source rocks and near source kitchen, its oil distribution is quasi-continuous, and the trap is transitional between the shaped trap and shapeless ones, while the typical continuous accumulation is occurred inside the source rocks and self-reservoired, its oil distribution is continuous, and the trap is insignificant and even unnecessary.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013