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Authigenic glauconite and siderite form under a limited range of well-documented geological and geochemical conditions. Because glauconite precursors need to remain at or near the sediment water surface for long periods of time in a setting where they can be repeatedly exhumed and shallowly buried, glauconite typically develops on the outer margins of continental shelves in areas of low sediment input. Based on these requirements, glauconite has traditionally been used as an indicator for transgressive sequences because transgressions tend to trap sediment on the continents. However, from a sequence stratigraphic standpoint, glaucony may be present in virtually any part of a depositional sequence due to remobilization. Glaucony can provide useful information for sequence stratigraphy only if variations in its abundance, physicochemical properties, and spatial/temporal characteristics are carefully documented. Siderite typically forms in one of two distinct environments: one characterized by strongly reducing conditions (methanogenic zone), and one under slightly reducing conditions (post-oxic zone). Methanogenic siderite is more common in continental and fresh-water lacustrine than marine deposits. Post-oxic conditions are commonly associated with marine environments exhibiting moderately low concentrations of organic matter and low sedimentation rates. Siderite is also frequently found in association with sequence boundaries where it occurs as a secondary cement below the lowstand surface of erosion (LSE). These restrictions on environment of origin provide information on the hydrologic regime and, by inference, the depositional and sequence stratigraphic setting of the host sediment in which these minerals are found. By examining the genetic significance of minerals such as glauconite and siderite, the origin of ambiguous, controversial, isolated marine sand bodies such as those discussed elsewhere in this volume may be clarified. This paper summarizes what is currently known about the chemical characteristics of these minerals and discusses generalized models of their distribution in a variety of sequence stratigraphic settings.

Ewing Bank 873 Field, developed from mid 1994 through 1995, produces from deepwater turbidite sands at an average depth of 11,000 TVD. Water depth at the platform is 775. In early 1996, pressure transient analysis of bottom hole pressure build-up surveys indicated increasing skins. Before pumping any type of remedial stimulation treatment, it was decided to test the recommended acid formulation in the lab. Acid corefloods were performed on composite samples from whole core using a typical 10% HCl/l .5% HF formulation. This acid formulation proved to be damaging as lab results indicated losses in permeability of up to 74%. A series of core floods were then performed on composite core samples to test the effects of various mud acid formulations, pre-flushes, post-flushes and acid additives. X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis were also performed to help in determining the cause and mechanism of the damage. Results indicated that the loss in permeability was associated with acid additives which were causing fines migration and with silica gels precipitated as a result of the aggressive reaction of HCl with zeolite (clinoptilolite) within the formation. An optimum treatment formulation was identified, consisting of 10% Citric/1.5% HF acid, with no additives except corrosion inhibitor. This treatment was successfully applied in the field to five producing wells. Aggregate rate increased from 7400 BOPD to 16,000 BOPD and productivity index (PI) increased from 4.78 to 12.73 BOPD/psi of drawdown. P. 693

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