Fractal Characteristics of Lacustrine Tight Carbonate Nanoscale Reservoirs

摘要

The complexity and heterogeneity of pore structure greatly affect gas-liquid accumulation and transport, and the fractal theory has been proven to be an effective approach for studying nanoscale reservoirs in shale, coal, and tight sandstones. However, researches on fractal characteristics and control mechanisms for the lacustrine tight carbonate have received little attention. Lacustrine tight carbonate samples from the Jurassic Da'anzhai Member in the Sichuan Basin in China were systematically investigated focusing on the fractal characteristics and control mechanisms of storage spaces, minerals, diagenesis, and paleoenvironments. The fractal dimensions can be separated into two different and valid parts including D-1 (2.515-2.785, average 2.652) and D-2 (2.424-2.562, average 2.485), and the correlation between them is negative rather than positive. The average pore diameters exhibit a positive correlation with D-1 and a negative correlation with D-2, and the storage space is positively correlated with D-2 and negatively correlated with D-1. Terrigenous minerals (e.g., quartz and clay) exhibit a positive correlation with D-2 and a negative correlation with D-1, whereas the effects of authigenic CaCO3 minerals (e.g., calcite and aragonite) are exactly opposite to those of terrigenous minerals, which is due to the diagenesis and the own characteristics of minerals. CaCO3 minerals can effectively change pore structures and fill the storage spaces (5 nm) by cementation, compaction, pressure-solution, recrystallization, and replacement, whereas terrigenous minerals have developed irregular intraparticle pores, interparticle pores, and microcracks. The low salinity and the humid (rainy) paleoclimate are favorable for the formation of terrigenous minerals (elements), whereas they are harmful to the formation of authigenic minerals (elements), which increases D-2 and reduces D-3. Additionally, paleoredox has a weak influence on the fractal dimensions.