克拉通盆地微古地貌恢复的构造趋势面转换法

为推算克拉通盆地弱构造变形区海相沉积期微幅度古地貌,提出了一种基于构造趋势面转换的沉积期微幅度古地貌推算方法。方法假设克拉通盆地的多阶段升降或扭转构造运动表现为构造趋势面的变动,但该变动并没有改变局部古高地与其周围地形的拟构造幅度。该方法的技术流程为,首先利用现今构造图及其趋势面求得拟构造幅度;然后将地层剥蚀线的平均走向近似于沉积期构造趋势面的走向,求得沉积期构造趋势面倾向;利用将今论古的思想借用现今潮坪相平均坡度作为沉积期构造趋势面的倾角;最后将沉积期构造趋势面与拟构造幅度相加即得沉积期古地形图。在此基础上,结合取心井的岩石学、白云石化类型,可以更好地确定海相沉积体系中各亚相或微相的分布范围。将此方法应用于苏里格气田东区下奥陶统马家沟组五段5亚段潮坪相碳酸盐岩地层的应用效果表明,研究区构造趋势面由NW—SE倾向变动到NE—SW倾向,其并没有改变沉积期局部古地貌与其周围地形的相对高低关系,该方法能够有效地刻画出潮坪相沉积期微幅度古地貌。沉积期水下微幅度古地貌隆起部位是控制该区潮坪相碳酸盐岩颗粒滩分布的关键因素,颗粒滩又是后期白云石化并发育为优质储层最有利的微相。因此,在准确刻画沉积期微幅度古地貌的基础上可以发现高产气井。该方法可以为其他克拉通盆地弱构造变形区沉积期古地貌研究提供有益的思路。

Structural trend surface conversion method for micro-amplitude paleotopographic restoration of cratonic basins

A method for estimation of sedimentary micro-amplitude paleotopography based on structural trend surface conversion has been proposed. This method is suitable for estimating marine sedimentary micro-amplitude paleotopography of weak tectonic deformation zone in a cratonic basin. An assumption is given that multiphase uplifting-subsiding or torsional structural movement in the cratonic basin is manifested as structural trend surface variation, which does not change the pseudostructural amplitudes of local paleotopographic highs and peripheral terrain. Technical procedure of this method is as follows:firstly, calculate pseudostructural amplitude using present-day structural map and its trend surface; secondly, calculate dip direction of sedimentary structural trend surface by approximating the average strike of formation denudation line to the strike of sedimentary structural trend surface; thirdly, according to the idea of uniformitaranism, the average gradient of present-day tidal-flat facies serves as the dip angle of sedimentary structural trend surface; lastly, structural trend surface plus pseudostructural amplitude can obtain the sedimentary paleotopographic map. On this basis in conjunction with petrographic characteristics and dolomitization type of cored wells, distribution scope of each subfacies or microfacies in the marine sedimentary system can be better determined. Application of this method in Lower Ordovician carbonate strata of tidal-flat facies in Submember 5 of Member 5 of Majiagou Formation in the eastern Sulige gas field shows that structural trend surface of the study area is changed from NW-SE to NE-SW, which does not change relative height between sedimentary local paleotopography and peripheral terrain, and that this method is able to effectively depict sedimentary micro-amplitude paleotopography of tidal-flat facies. Subaqueous micro-amplitude paleotopographic uplift during sedimentation period are a key factor controlling the distribution of tidal-flat-facies carbonate grain shoals in this area, while the grain shoals are the most favorable microfacies for late dolomitization and development of high-quality reservoirs. Therefore, high-production gas wells can be discovered on the basis of accurate depiction of sedimentary micro-amplitude paleotopography. This method can provide beneficial ideas for the research on sedimentary paleotopography of weak tectonic deformation zones in other cratonic basins.