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Christopher M. Prince1
(1) Petro Image, LLC, Columbia, SC

Abstract: Diagenetic evolution of packing flaws and well-packed porosity in reservoir sandstones

At deposition, sand grains tend to self-organize into clusters of well-packed grains separated by packing flaws. This characteristic structure imports a structure to the pore network that defines both single and multi-phase fluid behavior. By their nature, packing flaws form a network of large-well connected pores that are responsible for virtually all of the single-phase fluid flow and virtually all of the recoverable oil. Porosity within the clusters of well-packed grains averages one-half the size of packing flaws, and along with microporosity, tends to contain most of the irreducible water.

Diagenesis modifies this structure in two primary stages: during the initial compaction and dewatering stage of diagenesis, the well-packed clusters tend to consolidate. The porosity within packing flaws tends to be preserved while the average size of well-packed pores is reduced by approximately 20% and the relative proportion of well-packed porosity is reduced from approximately 45% down to 25%. Packing flaws should represent zones of mechanical weakness that are most susceptible to the effects of compaction. However, the reduction in pore space within the well-packed clusters must be balanced by the expulsion of water into the packing flaws. One explanation for the observed behavior is that the expulsion of water from the well-packed zones provides an increased hydraulic potential within the flaw network, which keeps the flawed zones from collapsing.

During the second, cementation stage of diagenesis, porosity within packing flaws is lost at approximately twice the rate of well-packed porosity. However, as there is a much greater proportion of flawed porosity at the start of cementation, flawed porosity persists and is dominant in low porosity sands. The continued dewatering of the rock (and any formations below) tends to preserve packing flaws oriented perpendicular to bedding.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana