川东北地区三叠系飞仙关组白云化流体的Ca/Mg值估算

摘要

作为川东北地区三叠系飞仙关组天然气气藏最重要储集岩的粒屑白云岩和结晶白云岩的成因始终是沉积和石油地质学家高度关注的研究热点之一.在总结川东北地区三叠系飞仙关组不同类型白云岩形成温度的基础上,通过已有的白云化流体Ca/Mg值和温度之间关系式获得了川东北地区三叠系飞仙关组白云化流体的Ca/Mg值的主要分布范围.结果表明:不同类型白云岩对应的白云化流体具有不同的Ca/Mg值,同时证实了三叠纪海水或与之有关的海源流体、三叠纪海水与大气淡水混合流体的Ca/Mg值都符合在40～130℃区间、甚至更高温度下白云化(白云石沉淀)作用的要求,微晶白云岩和具原始结构的粒屑白云岩可能与地表—近地表的低温、小Ca/Mg值混合流体中相对快速的白云石沉淀作用有关,并形成了相对细的他形白云石；结晶白云岩(包括结构类似的过渡岩石类型、白云石胶结物)可能与埋藏的高温、大Ca/Mg值三叠纪海水或与之有关的海源流体中相对缓慢的白云石沉淀作用有关,并形成了相对粗的自形白云石.白云化流体的Ca/Mg值估算,可望为川东北地区三叠系飞仙关组优质储层白云岩的成因研究提供一些有价值的基础资料.%Grainy dolomite and crystalline dolomite are the most important natural gas reservoir rocks of the Triassic Feixianguan Formation in the northeastern Sichuan basin, but their origins have long been the focus of research for sedimentologists and petroleum geologists. Based on the formation temperatures of different types of dolomites in the Triassic Feixianguan Formation, and the relationship between the temperatures and Ca/Mg ratios of the dolomitizing fluid, this paper obtained the main distribution pattern of Ca/Mg ratios of dolomitizing fluids. The results show that the Ca/Mg ratios of the dolomitizing fluids correspond to different types of dolomites. It is also confirmed that the Ca/Mg ratios of Triassic seawater or related sea-derived fluids, and of mixing fluids of both Triassic fluid and meteoric water are in accord with requirements of dolomitization or dolomite precipitation at temperature ranges from 40°C to 130°C , or even higher temperature. The micrite dolomite and grainy dolomite (with original texture) could be related to relatively rapid precipitation at a low temperature from the surface or near-surface mixing-fluids with low Ca/Mg ratios, and these dolomites are relatively fine and anhedral. The crystalline dolomite (including the transitional rock types with the similar texture and dolomite cement) could be related to relatively slow precipitation at high temperature from the burial Triassic seawater or related fluids with high Ca/Mg ratio, and these dolomites are relatively coarse and euhedral. The Ca/Mg ratio estimation of the dolomitizing fluids will provide some valuable basic data to the studies of the high-quality dolomite reservoirs in the Triassic Feixianguan Formation, northeastern Sichuan basin.