【作者】 胡作维；

【导师】 黄思静；

【作者基本信息】 成都理工大学 ， 矿物学、岩石学、矿床学， 2010， 博士

【摘要】 川东北地区三叠系飞仙关组天然气储量规模之大,深埋藏条件下储层质量之好,在国内外海相碳酸盐岩地层中十分罕见。川东北地区飞仙关组天然气的勘探离不开对白云岩的勘探,只要找到这些代表优质储集岩的优势岩相—白云岩,那么天然气的勘探就有希望获得大的突破。目前,川东北地区三叠系飞仙关组碳酸盐岩白云化作用并最终形成优质储层的成岩机制成为了人们近年来最为热点的研究领域之一。本文在详细的岩心观察与描述、野外剖面实测、样品采集基础上,对川东北地区三叠系飞仙关组白云岩开展了较为系统的岩石学、地球化学、白云化流体温度与来源、白云化系统与白云化机制等方面研究。川东北地区飞仙关组主要的白云岩(石)类型可以归并为3个大类:原始结构保存的白云岩(包括微晶白云岩、粒屑白云岩)、原始结构不保存或保存较差的结晶白云岩和作为胶结物出现的孔洞白云石。在纵向分布上,较为有利于储层发育的结晶白云岩、粒屑白云岩主要分布在川东北地区飞仙关组1段和2段;在区域分布上,飞仙关组1段和2段的白云岩是非层控的、具有明显的区域分带性,而飞仙关组4段的白云岩具有显著的层控性,不同地区都有分布。川东北地区飞仙关组不同地层段灰岩样品以方解石矿物为主,多数样品所具有的Mn含量只分布在500ppm以下、Sr含量则在500ppm以上。总体上看,川东北地区三叠系飞仙关组海相碳酸盐的锶和碳同位素演化曲线与全球同期海水的锶和碳同位素组成和演化趋势具有较好的一致性,显示全球事件对海水锶和碳同位素组成的控制作用,而氧同位素已经与大气淡水(或其它成岩流体)发生了交换,很难反映其原有的氧同位素组成。飞仙关组白云岩的锶同位素组成都不同程度地高于其相应的同期海水的锶同位素组成,但白云化流体仍与同期或近同期海水有着强烈的亲缘关系;飞仙关组1段和2段白云岩主要继承了海源流体中的碳,而飞仙关组4段中一些具有很高碳同位素组成的白云岩则与细菌硫酸盐还原作用(BSR)有关;飞仙关组1段和2段白云岩的氧同位素组成基本接近,显示它们的白云化流体氧同位素组成和白云化温度均基本接近,而飞仙关组4段白云岩则与经强烈蒸发作用后具有更高的氧同位素组成的高盐度泻湖水有关。川东北地区飞仙关组少数可以进行流体包裹体测温的较大白云石晶体(主要是细晶、中晶白云石)的白云化流体可能出现在100℃—130℃的温度区间内,与多数方解石沉淀的温度相一致,属于高温白云石(高温白云化流体);而流体包裹体不发育或体积小的其它较大白云石晶体(包括了粉晶、细晶、中晶以及它们混合组成的不等晶)的白云化流体可能出现在小于100℃的温度区间内,属于低温白云石(低温白云化流体)。多数川东北地区飞仙关组白云岩(石)的计算温度分布在40℃—140℃(氧同位素内部计温法)或者60℃—100℃(氧同位素外部计温法)范围内,尤其集中在<100℃的范围内。这些计算温度的高温区间显然与流体包裹体测温数据相吻合,显示氧同位素计温法的计算结果具有一定的可靠性,同时这些计算结果也在某种程度上进一步支持这些白云石沉淀流体(白云化流体)很可能主要出现在40℃—100℃的低温区间内。川东北地区飞仙关组白云岩的白云化流体的Ca/Mg比更可能分布在<1.17的范围内(40℃—100℃的温度区间),包括三叠纪海水或与三叠纪海水有关的海源流体均可以满足这一要求。飞仙关组结晶白云岩和原始结构保存的粒屑白云岩均具有较低的Mn、Fe含量特征,而飞仙关组微晶白云岩则具有相对高的Mn、Fe含量;飞仙关组白云岩的Sr含量均大致分布在200ppm以下。飞仙关组结晶白云岩的碳来源与原地(近源)或远源碳酸盐溶解迁移有关;粒屑白云岩的碳来源与近同期海水中碳有关;微晶白云岩的碳来源较复杂,大部分可能受到了大气淡水中轻碳的混合影响,小部分则来源于与细菌硫酸盐还原作用(BSR)有关的富重碳(13C)的准同生期孔隙水。结晶白云岩和孔洞白云石更可能与深埋的相对高温白云化流体来源有关,原始结构保存的粒屑白云岩、微晶白云岩则更可能与近地表的相对低温白云化流体来源有关,部分微晶白云岩可能受到了后期白云石重结晶作用过程中氧同位素再平衡的影响。不同类型白云岩(石)的87Sr/86Sr比值显著大于川东北地区飞仙关组同期海水的87Sr/ 86Sr比值,但海水或相应的海源流体仍然是最为重要的白云化作用流体。川东北地区飞仙关组白云化系统可以归纳为两个在地球化学上存在着明显差异的白云化系统:开放白云化系统、封闭白云化系统。飞仙关组结晶白云岩和孔洞白云石很可能与封闭白云化系统中嘉陵江期高度蒸发浓缩的海源流体有关,对应着非同期海源流体来源的埋藏白云化模式;飞仙关组粒屑白云岩很可能与开放白云化系统中同期或近同期侧向或垂向泻湖蒸发海水有关,对应着准同生或较浅埋藏条件下浅部潜流环境中蒸发海水来源的渗透回流白云化模式;飞仙关组微晶白云岩的白云化流体很可能与开放白云化系统中大气淡水介入(混合)的海水—大气水混合流体有关,对应着准同生条件下潜流环境中海水—大气水混合的蒸发泵白云化模式,飞仙关组4段局部地区微晶白云岩可能对应着准同生期强还原环境下微生物(细菌)白云化模式。更多还原

【Abstract】 There are giant reserves of natural gas and high-quality reservoir rocks in the deeply buried Triassic Feixianguan Formation, Northeastern Sichuan Basin, which is rare in the domestic and international marine carbonate reservoirs. The natural gas exploration for the Triassic Feixianguan Formation, Northeastern Sichuan Basin relates to the high-quality dolomite reservoirs, which are represented as advantage reservoir facie. If the high-quality dolomite reservoirs could be found, and then the natural gas exploration will be hopeful. The dolomitization and diagenesis of the high-quality dolomite reservoirs in Triassic Feixianguan Formation, Northeastern Sichuan Basin have become the hot research field in recent years. Based on detailed core description, field work and sample collection, the research of petrography, geochemistry, temperature and origin of dolomitizing fluids, dolomitizing system and mechanism was systematiclly carried out in this study.The main dolomite types in Triassic Feixianguan Formation include micrite dolomite, grainy dolomite (with good original texture), crystalline dolomite (poor or non-original texture) and dolomite cement. Vertically, crystalline and grainy dolomites are more likely to distributed in the Member 1 and 2 of Feixianguan Formation, and they are non-strata bound and local distribution, whereas dolomites in the Member 4 of Feixianguan Formation are significantly strata bound and regional distribution in the Northeastern Sichuan Basin.Limestone samples in the Triassic Feixianguan Formation are dominated by calcite mineral, of which Mn contents are less than 500ppm and Sr contents are more than 500ppm. The strontium and carbon isotopic evolution curve of marine carbonate in the Triassic Feixianguan Formation are rather consistent with the data from global coeval seawater, indicating that the global geological events control the evolution of strontium and carbon isotopic composition of seawater, whereas oxygen isotopes have exchanged with meteoric water or other diagenetic fluids and could not represent the original composition of the seawater. The strontium isotopic composition of dolomites in the Triassic Feixianguan Formation is higher than that of the corresponding coeval seawater, but dolomitizing fluids is still close to the seawater. The dolomites in the Member 1 and 2 of Feixianguan Formation inherited the carbon from seawater, whereas a few dolomites in the Member 4 of Feixianguan Formation with heavy carbon isotope signatures relate to bacterial sulfate reduction (BSR). The oxygen isotopic composition of dolomites in the Feixianguan Formation are almost close to each other, indicating the similar oxygen isotope composition of dolomitizing fluids and dolomitizing temperatures, but the dolomites in the Member 4 of Feixianguan Formation with higher oxygen isotope composition relate to high-salinity lagoon water after the strong evaporation.The homogenization temperatures of some fluid inclusions in the large dolomite crystals (mainly fine to medium grain) indicate that hose dolomite crystals may precipitate at 100℃to 130℃, which are consistent with the temperatures of main calcite precipitation, so they belong to the high-temperature dolomitizing fluids. The other large dolomite crystals (including powder to medium grain, and their mixed grain), which has few fluid inclusions or only small size ones, may precipitate in less than 100℃, so they belong to low-temperature dolomitizing fluids. The calculated temperatures of most dolomites in the Triassic Feixianguan Formation are at temperatures of 40℃-140℃(within the internal oxygen isotope geothermometer) or 60℃-100℃(within the external oxygen isotope geothermometer), and most of them are particularly at <100℃. The high-temperature zone of the calculated temperatures is significantly consistent with homogenization temperature of fluid inclusions, indicating calculated temperatures of the oxygen isotope geothermometer may provide a reliable estimation of dolomitizing temperature. The calculated temperatures of the oxygen isotope geothermometer further support that dolomites may precipitate almost in the low-temperature range at 40℃-100℃.The Ca/Mg ratios of the dolomitizing fluids in the Triassic Feixianguan Formation are more likely less than 1.17 at temperatures of 40℃-100℃, and the Triassic seawater or Triassic marine fluids were sufficiency to this ratio. The grainy dolomites (with good original texture) and crystalline dolomites (with poor or non-original texture) contain low Mn, Fe contents, whereas micrite dolomites contain high Mn, Fe contents. The Sr contents of dolomites in the Triassic Feixianguan Formation are generally less than 200ppm. The carbon sources of crystalline dolomites in the Feixianguan Formation relate to in-situ or distal migration of the dissolved carbonate, and carbon sources of grainy dolomites in the Feixianguan Formation relate to coeval or non-coeval seawater. The carbon sources of micrite dolomites in the Feixianguan Formation are more complex. Most of them are impacted by the mixture of light carbon isotope from meteoric water, and a few of them are impacted by pore water with heavy carbon isotope from bacterial sulfate reduction (BSR). The crystalline dolomites and dolomite cements are more likely related to the deeply buried dolomitizing fluids with relatively high temperature, and grainy dolomites (with good original texture) and micrite dolomites are more likely related to the dolomitizing fluids near surface with relatively low temperature. Some micrite dolomites are susceptible to further oxygen isotopic re-equilibrium in the recrystallized process. The 87Sr/86Sr ratios of dolomites in the Triassic Feixianguan Formation are significantly greater than that of coeval seawater, but the seawater is still the most important dolomitizing fluid.The dolomitizing systems in the Triassic Feixianguan Formation can be summarized in two significantly different types by geochemistry: open system and closed system. The crystalline dolomites and dolomite cements are more likely associated with closed system that relate to high-salinity evaporative seawater from the Jialingjiang Formation, and correspond to burial dolomitization by non-coeval seawater. The grainy dolomites are more likely associated with open system that relate to lateral or vertical high-salinity evaporative lagoon water, and correspond to penecontemporaneous or shallow burial dolomitization (seepage reflux) by evaporative seawater. The micrite dolomites are more likely associated with open system that relate to mixture of meteoric water and seawater, and correspond to penecontemporaneous evaporation-pump dolomitization in the mixing-zone, whereas a few micrite dolomites in the Member 4 of Feixianguan Formation may correspond to penecontemporaneous microbes (bacteria) dolomitization in the anoxic reducing condition.更多还原