方解石充填物对于深层古岩溶洞穴保存的独特意义——以塔河地区奥陶系为例

Distinctive Significance of Calcite Fillings for Preservation of Deep Buried Karst Paleocaves: Taking the Ordovician System in Tahe Area for Example

塔河油田奥陶系古岩溶洞穴埋藏深度之大在世界范围内罕见,但对其为何没有垮塌殆尽的原因却知之甚少。为了探究其保存机制,对比了Fort Worth盆地奥陶系深埋洞穴,发现大量缝、洞充填方解石是塔河地区洞穴中常见且特有的充填物。在统计和综合分析缝洞方解石充填物地球化学数据的基础上,以洞穴充填方解石成因类型为切入点,详细分析了构造运动背景、热液活动、油气充注、方解石的产状以及洞穴受力情况,通过与Fort Worth盆地深埋洞穴埋藏史的对比和分析,提出塔河地区深埋岩溶洞穴的一种保存机制。研究区岩溶洞穴方解石主要是长期浅埋阶段,以大气淡水成因为主,海水成因为辅形成,深埋过程还受到了溶蚀性流体(岩浆热液和有机酸)后期改造。方解石充填物增加了洞穴的抗压性,岩溶洞穴内部充填的方解石在埋藏过程中支撑了洞穴;岩溶洞穴顶部裂纹中充填的方解石还"愈合"了洞穴周边的裂纹,强化了洞穴的顶板,增加了洞穴整体的稳定性。充填的方解石在垮塌前占据了部分洞穴空间,在埋藏后,高温有机酸、CO_2、H_2S和岩浆热液沿着深大断裂等通道对奥陶系碳酸盐岩改造溶蚀作用明显,也对处于优势通道和良好储集性能的洞穴充填方解石进行了改造,恢复了部分洞穴空间。因而认为地表、近地表方解石充填过程是碳酸盐岩洞穴型储层的"保持性成岩作用"。

The buried depth of karst paleo-caves in Tahe oilfield, Tarim basin is deeper than others throughout the world. But little is known about the reason why it did not collapse. In order to explore the preservation mechanism of Ordovician deep buried karst paleocaves in Tahe oilfield, by comparing burial history with the buried caves of Ordovician in Fort Worth basin, we noticed that there are a large number of calcite fillings in caves and fractures in Tahe oil-field. Based on statistical and comprehensive analysis of geochemical data, taking calcite genesis as the pointcut, the tectonic background, volcanogenic hydrothermal activities, oil and gas filling, occurrence of calcite and cave stress distribution are analyzed and a preservation mechanism of deep buried caves is put forward. Calcite precipitated in the near-surface filled part of the caves and rock fractures. Calcite is mainly precipitated in atmospheric fresh water environment ,and a small amount of calcite formed under seawater environment. Calcite was also reconstructed by dissolution fluid （magmatic hydrothermal and organic acid） during burial process. They shared the overlying strata pressure in the process of strata subsidence, so that the caves have not been fully filled by collapsed breccia. Calcite added the pressure resistance of caves. Calcite filled in fractures of cave ceiling ＂healed＂ cracks near the cave, which strength-ened the cave roof, increased stability of caves. Before the cave collapse, calcite occupied part of the cave space. When caves were deeply buried, dissolution fluids corroded part of calcite in caves preferentially through the cave system and fracture system. Therefore, filled caves have been recovered partly. So, the calcite filling in surface and near-surface is “retention diagenesis” of carbonate cavern reservoir.