Gas hydrate structures and C1-C5 hydrocarbons at individual sites in the Gulf of Mexico

Gas hydrate samples from various locations in the Gulf of Mexico (GOM) differ considerably in their microstructure. Distinct microstructure characteristics coincide with discrete crystallographic structures, gas compositions and calculated thermodynamic stabilities. The crystallographic structures were established by X-ray diffraction, using both conventional X-ray sources and high-energy synchrotron radiation. The microstructures were examined by cryo-stage Field-Emission Scanning Electron Microscopy (FE-SEM). Good sample preservation was warranted by the low ice fractions shown from quantitative phase analyses. Gas hydrate structure II samples from the Green Canyon in the northern GOM had methane concentrations of 70-80% and up to 30% of C2-C5 of measured hydrocarbons. Hydrocarbons in the crystallographic structure I hydrate from the Chapopote asphalt volcano in the southern GOM was comprised of more than 98% methane. Fairly different microstructures were identified for those different hydrates: Pores measuring 200-400 nm in diameter were present in structure I gas hydrate samples; no such pores but dense crystal surfaces instead were discovered in structure II gas hydrate. The stability of the hydrate samples is discussed regarding gas composition, crystallographic structure and microstructure. Electron microscopic observations showed evidence of gas hydrate and liquid oil co-occurrence on a micrometer scale. That demonstrates that oil has direct contact to gas hydrates when it diffuses through a hydrate matrix.取自墨西哥湾（Gulf of Mexico, GOM）不同区域的天然气水合物（Gas hydrate）样品，其微观结构存在显著差异。独特的微观结构特征与特定的晶体结构、气体组分以及计算得到的热力学稳定性一一对应。 本次研究通过X射线衍射（X-ray diffraction）确定了样品的晶体结构，所用辐射源涵盖常规X射线源与高能同步辐射光；微观结构则借助带冷冻样品台的场发射扫描电子显微镜（Field-Emission Scanning Electron Microscopy, FE-SEM）进行观测。定量物相分析结果显示样品冰占比极低，确保了样品保存状态良好。 取自墨西哥湾北部绿峡谷（Green Canyon）的II型结构天然气水合物样品，其甲烷浓度为70%~80%，测得的烃类组分中C2~C5烷烃占比最高可达30%；而取自墨西哥湾南部查波波特沥青火山（Chapopote asphalt volcano）的I型结构天然气水合物，其烃类组分中甲烷占比超过98%。这两类不同的水合物展现出截然不同的微观结构：I型结构天然气水合物样品中存在直径200~400 nm的孔隙，而II型结构天然气水合物则未观察到此类孔隙，取而代之的是致密的晶体表面。本文还围绕气体组分、晶体结构与微观结构，探讨了水合物样品的稳定性问题。 电子显微镜观测结果显示，在微米尺度下存在天然气水合物与液态油共存的现象，这表明当油分透过水合物基质扩散时，会直接与天然气水合物发生接触。