欠压实泥岩顶底板形成机理及其对油气运移的影响

欠压实泥岩顶底板是指比中部欠压实泥岩较致密的泥岩表层,是趋于正常压实的泥岩层,发育于低渗透率的厚层泥岩的上、下部位。以低速渗流定律为理论基础,从微观机理上分析欠压实泥岩顶底板的形成过程,提出穿过欠压实泥岩顶底板流体运移阻力的计算公式。当含顶底板泥岩作为盖层时,分为连续沉积型、抬升型和压裂型3种。压裂型泥岩盖层的封闭能力等于底板排替压力与达到顶板破裂的剩余压力之和,其他两种盖层的封闭能力就等于盖层顶底板的排替压力之和。当含顶底板泥岩作为源岩时,在连续型源岩中,油气排放的方向主要取决于泥岩顶底板渗透性地层的发育程度;在压裂型源岩中,大量油气沿裂缝向上排烃。此外,源岩的排烃方向还受断层的控制。通过欠压实泥岩顶底板形成机理的研究,有助于评价泥岩盖层的封烃能力,了解泥质烃源岩油气初次运移的方向。

Formation Mechanism of the Undercompacting Mudstone's Roof and Floor and Their Influence on Hydrocarbon Migration

Undercompacting mudstone’s roof and floor(UMRF) are the mudstone surface layers that are tighter than the central undercompaction,and are close to the normal compacting mudstone.UMRF develops in upper and lower parts of thick-layered mudstone with low permeability.This paper,by the Laws of Low-speed Flow,analyzed formation process of the UMRF from the microscopic mechanism,and proposed the resistance force formula of the fluid passing UMRF.The mudstone with the UMRF as a cap rock,can be divided into three types: the continuous sedimentary type,the uplift type and the fracture type.The sealing ability of the fracture type mudstone cap rock is equal to the floor’s displacement pressure plus the roof’s overpressure when it breaks down.The other two types of mudstone cap rocks’ sealing abilities are equal to the floor’s displacement pressure plus the roof’s displacement pressure.When the mudstone with the UMRF as a source rock,in the continuous type,the expulsion direction of the oil and gas is controlled by the permeability of UMRF;In the fracture type,a large number of oil and gas migrated upward along the cracks.In addition,the expulsion direction of the source rock is also controlled by the fault.The research of UMRF’s formation mechanism can help to evaluate the sealing ability of the mudstone cap rock and to understand the muddy source rock’s expulsion direction in the primary migration.