Cementation of oilfield sandstones

Introduction

• How does water move in the subsurface within hydrocarbon basins?
• How does this affect the reservoir quality of the sandstones?

During burial of an oilfield reservoir sandstone, new minerals crystallise in the porosity between the sand grains. These minerals adversely affect the quality of sandstone as an oil reservoir, but also hold a fossil record of the ancient water conditions in which they grew. Micro-analysis of the individual crystals surrounding oilfield sand grains can be undertaken using mass-spectrometers linked to electron beams, ion beams, or laser light. The record of ancient water in these minerals is held as subtle changes of chemical composition, or as isotope ratios. This enables interpretation of the palaeo-hydrogeology of the deep subsurface during burial of sandstone. The mineral record shows that deep below ground there was a race between oil movement and crystal growth. If oil arrived in the sand before crystal growth had filled the pore-space, then a good oilfield results.

The cartoons show cross-sections through three different oilfields, with arrows showing the inferred directions of water movement. Each oilfield has had different hydrogeologies proposed to explain the origin of the cementing minerals. This project will apply new micro-analytical technologies to gain access for the first time to the very detailed mineral record of deep water flow in each case.

1. The Magnus oilfield may have had static water, OR local water circulation during the filling by oil.
2. The Sohlingen Graben may have experienced flow of hot fluids up faults, OR may have simply trapped oil a lot longer ago than currently supposed.
3. The Embla oilfield may have expelled highly pressured waters laterally through the sandstones, OR the waters may have been derived from the crystalline crust beneath.

Sandstones from the 3.5km deep Magnus oilfield in the North Sea are being investigated. This is a Scanning Electron Microscope picture of quartz grains within the sandstone (click here for on the image for a bigger picture). Also visible is the porosity between sand grains, with small connecting holes between the pores. The ragged grain near the front is a partly dissolved feldspar. The smooth flat crystals visible are the outside surfaces of quartz which has grown as a cement during burial of the sandstone, gradually filling in the porosity. These grains will be sliced flat and analysed by ion microprobe at Edinburgh University (NERC facility), to investigate the detailed isotope and trace element signatures incorporated during their growth. This information enables the history of ancient water movement to be reconstructed. View about 400 microns across

Sandstones from the Brae oilfield in the North Sea are here imaged, as a thin flat and polished slice, in Cathode Luminescent light on a Scanning Electron Microscope . This shows that the rounded outlines of quartz grains in the original sand (pale greys), have been grown over by angular quartz cement, which has formed during burial. A sequence of growth zones within the quartz cement can be seen (click here or on the image for a bigger picture). Round craters about 50 microns across in the quartz grains and in the cement show positions where the ion microprobe beam at Edinburgh University has analysed the sandstone. The numbers are the oxygen isotope readings measured by the instrument. The variation in these readings is due to changes in the deep groundwater filling this porosity whilst the cement was growing during burial of the sandstone. Interpretation of such analyses enables a palaeo-hydrogeology of the sandstone to be reconstructed.

Aims of this project include:

• To reduce the size of analytical craters to 20 microns.
• To determine how microscopic analyse scan be scaled-up to represent an entire sandstone.
• To understand how sandstones became cemented, and so improve prediction of porous, and poorly-cemented, sandstones which form good quality oilfields.