Abstract

The integration of petrographic analyses of cores and thin sections, petrophysical measurements, and well logs demonstrates that variations in acoustic impedance in the Malampaya buildup (upper Eocene to lower Miocene, offshore northwest Palawan) are related to vertical changes in porosity and pore type, which are dominantly controlled by diagenetic processes. The Nido Limestone was subdivided into 10–50-m (30–150-ft)-thick units characterized by specific diagenetic patterns and petrophysical properties (diagenetic units). The alternation between tight and porous diagenetic units is mainly controlled by meteoric diagenesis (leaching and pedogenesis) and by late-burial cementation and leaching. Well-to-seismic ties show that the main seismic reflectors within the buildup interior reflect the boundaries between diagenetic units. Most of the negative amplitude reflectors are related to unconformities, whereas positive amplitude reflectors have a more questionable chronostratigraphic value and may represent the bases of cemented lenses that can crosscut time lines. Comparison with other southeast Asian Tertiary buildups indicates different origins of seismic reflections related to distinct patterns of diagenetic evolution. The identification of such seismodiagenetic units and of their controlling factors is of paramount importance for the architecture and the petrophysical characterization of carbonate reservoirs.

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