关键词: “饼状”缝, 低角度缝, 不等距缝, 须家河组二段, 新场构造带

Abstract:

A distinct category of fractures, characterized by a substantial proportion, shallow angle, and contentious origin, commonly referred to n as “cake-like” fractures or “laminar cake” fractures within the second member of Xujiahe Formation of Xinchang structural belt, has been chosen as the research target. This study employs meticulous core observation, precise thin section analysis, and a comprehensive comparative assessment of both macroscopic and microscopic traits. The primary objective is to classify these “cake-shaped” fractures for the first time and elucidate their corresponding lithofacies attributes and genesis. The discussion regarding the oil and gas geological significance of these “cake-like” fractures is rooted in the distinctions in gas-bearing properties. The analysis reveals the presence of three distinct types of “cake-like” fractures: thin layer “shortcake” fractures, medium-thick layer unequal spacing fractures and medium-thick layer isometric fractures. The thin layer “shortcake” fractures manifest within isolated distributary channels with coarse-grained textures and a high quartz composition. Conversely, the medium-thick layer unequal distance fractures are evident in interbedded distributary channels exhibiting fine, medium-grained, and coarse-grained textures with a notable feldspar content. The medium-thick layer isometric fractures occur within distributary channels or estuarine bar resulting from the deposition of alternating fine and medium-grained calcareous-rich sand and calcareous-poor sand. The initial two fracture types can not only improve reservoir permeability, but also intensify the dissolution effect, leading to higher gas content, while the latter type only increases the permeability and has a limited impact on the matrix pores, resulting in lower gas content. The “cake-like” fracture phenomenon represents a composite fracture formation influenced by sedimentary environment, tectonic stress, and differential diagenesis. It defies a simple classification as a tectonic shear fracture, sedimentary bedding fracture, or stress unloading fracture. The areas with high structural position, well-developed pie fractures and favorable reservoir properties, augmented by effective fracture network fracturing technology, present promising prospects for future exploration endeavors.

Key words: “cake-like” fracture, low dip-angle fracture, unequal spacing seam, second member of Xujiahe Formation, Xinchang structural belt