Isotope Investigation of Mass Transfer in a Sedimentary Basin

我们建议研究圣华金盆地的流体-岩石相互作用和传质速率。这项工作将测试沉积盆地的流体通量模型，包括：压实驱动的流动、热驱动的对流和大气补给。我们的方法是使用87Sr/86Sr和143Nd/144Nd比率作为同位素示踪剂，结合我们目前对成岩反应的理解，来确定盆地中钙的流动性。我们将比较钙汇（碳酸盐和烟云母胶结物）和潜在的钙源，包括：海洋生物成因碳酸盐、碎屑钙蒙脱石和钙斜长石。初步研究表明，钙斜长石最有可能是方解石胶结物中钙的来源。我们相信这项研究将为盆地成岩过程提供一个独特的见解，并且由于该地区的地质条件很好，我们相信它是研究这一类型的理想选择。我们的成功将取决于能够识别每个潜在来源的同位素“指纹”，能够可靠地测量胶结物中的同位素比率，并能够预测反应的时间。到目前为止，我们的工作有力地表明，Sierran斜长石是唯一一个比值足够低的Sr同位素来源，可以解释我们详细研究过的中新世油藏North Coles Levee胶结物中观察到的Sr同位素。此外，斜长石的溶蚀作用将唯一地解释在盆地逐渐改变的现代孔隙水中观察到的下降比率。我们建议将这些在北科尔斯大堤的研究扩展到整个流域。为了验证斜长石来源Ca的假设，我们建议研究四个长石蚀变不同的区域：1)未蚀变的斜长石区域，2)丰富的斜长石溶蚀，3)丰富的斜长石钠长石化，4)钾长石溶蚀但没有斜长石蚀变。在每个区域，我们将测量胶结物和包括斜长石在内的潜在钙源的同位素值。还将测量孔隙水，以监测流体-岩石相互作用对锶同位素比的影响。在可能的情况下，将计算每个区域的传质率。最后，通过与其他小组的合作研究，我们计划将我们的传质估计值与各种水文模型预测的传质估计值进行比较。我们相信这个受控的、多学科的项目对于理解整个盆地范围内的传质有很大的潜力。

We propose to investigate fluid-rock interaction and mass transfer rates in the San Joaquin basin. The work will test models of fluid flux in sedimentary basins including: compaction driven flow, thermal driven convection, and meteoric recharge. Our approach is to use 87Sr/86Sr and 143Nd/144Nd ratios as isotopic tracers, in combination with our present understanding of diagenetic reactions, to determine calcium mobility in the basin. We will compare calcium sinks (carbonate and laumontite cements) with potential calcium sources including: marine biogenetic carbonate, detrital Ca-smectite, and Ca-plagioclase. Preliminary work suggests that Ca-plagioclase is the most likely source for calcium in calcite cements. We believe that this study will offer a unique insight into basin diagenetic processes, and owing to the well-constrained geology in this area, we believe it is ideal for a study of this type. Our success will depend on being able to recognize isotopic "fingerprints" for each potential source, to be able to reliably measure isotopic ratios in cements, and to be able to predict timing of reactions. Our work to date strongly indicates that Sierran plagioclase is the only Sr isotopic source with ratios low enough to account for those observed in cements at North Coles Levee, a Miocene reservoir we have examined in detail. In addition, dissolution of plagioclase would uniquely account for the decreasing ratios observed in progressively modified modern pore waters of the basin. We propose to extend these studies at North Coles Levee into a basinwide scale. To test the hypothesis of plagioclase sourced Ca, we propose to examine four areas of varying feldspar alteration: 1) areas with unaltered plagioclase, 2) with abundant plagioclase dissolution, 3) with abundant plagioclase albitization, and 4) with K-feldspar dissolution but without plagioclase alteration. In each area we will measure isotopic values for cements and potential Ca sources including plagioclase. Pore waters will also be measured to monitor the effects of fluid-rock interaction on Sr isotopic ratios. Mass transfer rates will be calculated for each area where possible. Finally by cooperative research with other groups we plan to compare our estimates of mass transfer with those predicted by various hydrologic models. We believe this controlled, multi- disciplinary project has great potential for understanding mass transfer on a basin-wide scale.