Prodelta, delta-front, and delta-plain sandstones and shales were deposited in a wave-dominated lobate delta in the Rio Escondido basin during Campanian and Maestrichtian time. The

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composite diagenetic sequence based on petrographic analyses of over 300 thin sections from sandstone samples from these environments consists of: (1) bioturbation of sediment and compaction of ductile grains before cementation; (2) infiltration of minor amounts of pedogenic illite and precipitation of chlorite and minor amounts of smectite; (3) cementation by euhedral, syntaxial quartz overgrowths; (4) extensive poikilotopic calcite cementation; (5) widespread dissolution of calcite; (6) precipitation of kaolinite as patches in some of the dissolution pores; (7) replacement of calcite and framework grains by iron-rich carbonates (ankerite-ferroan dolomite, ferroan calcite, siderite). The relative intensities of these diagenetic events, and thus porosity and permeability, are controll d by and vary with the environment of deposition of the sandstones. The two main factors controlling the differences in diagenetic features within the environments are (1) original composition of the sands, and (2) geometry of the sandstone bodies.

Prodelta siltstone and very fine-grained sandstone, deposited as thin frontal-splays, are enclosed in thicker sequences of marine shale. These rocks (dominantly feldspathic litharenite and lithic arkose) typically have the lowest porosity (x = 4%) and permeability. (x = 1 md) of the three environments. Primary porosity was destroyed early by extensive bioturbation and by widespread calcite cementation (x = 23%). The thicker, impermeable shale above and below the thin, lenticular sandstone bodies served as barriers to subsequent fluid migration, thus inhibiting any creation of secondary porosity by dissolution and any later diagenetic changes.

Channel-mouth-bar and sheet-sand deposits of the delta-front facies are generally more quartz-rich and thus, due to a greater stability of the quartz grains, underwent the least amount of compaction. Consequently, calcite cementation was widespread; later dissolution of calcite produced secondary porosity up to 28% and permeability up to 557 md. These fine to medium-grained sandstones (dominantly feldspathic litharenite, lithic arkose, and subarkose) have the highest porosity (x = 18%) and permeability (x = 81 md) of rocks from the three facies.

Delta-plain fine to medium-grained sandstone has intermediate porosity (x = 8%), but low permeability (x = 1 md). These sandstones (primarily feldspathic litharenite) have the highest concentration of sedimentary and volcanic rock fragments, which were squashed during compaction and altered during burial. Thus, only a relatively small amount of pore space remained for calcite cementation. Kaolinite cement filled in most of the secondary porosity created by dissolution of early formed calcite.

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