THOMAS, ANDREW R.¹, SUSAN D. ANDERSON³, GREG P. PEPIN¹, WILLIAM M. DAHL², and ROBERT M. MOSS^1
1Texaco EPTD, Houston, TX
²Texaco N. America Prod., New Orleans, La
³Univ of Texas, Austin, TX

Abstract: Stylolitization and Resulting Porosity Reduction in the Norphlet Formation, Offshore Alabama

Stylolites are common in the Norphlet Formation, and average nearly one/foot through the gas column in the study well (Mobile Area Block 872#1). Without exception, all stylolites have an adjacent mineral cemented halo (up to 2 cm thick). Combined petrographic and catscan analyses show that halos contain elevated volumes of quartz cement and K-spar cement (8% & 6% respectively) as compared with areas outside of the halo. Porosity decreases dramatically within the cemented halo, usually dropping at least 6% to near zero. The catscans show that the cemented halos follow the host stylolite even where sutures significantly offset the stylolite seam. This evidence, combined with the lack of significant variance of cement abundance in non-stylolitized rock, indicates that the mineral cement halos are back-precipitated products of stylolite pressure solution.

Norphlet stylolitization and quartz cementation occurred during late burial within a standing gas column. We interpret that available S_w was quickly supersaturated with respect to mineral cements, which led to mineral precipitation adjacent to the stylolites. Additionally, stylolite halo thickness shows a strong inverse trend with respect to depth. This observation implies that the downsection increase in S_w found in normal capillary relationships provides an increasing fluid volume that is progressively more difficult to supersaturate. Halo precipitation is therefore strongly influenced by ambient water saturation as well as stylolite intensity, fluid chemistry, and fluid movement.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas