The change in the price of crude oil over the past month (nil) and the change in Saudi's crude oil export market share (nil) is unlikely to have explained the most recent gain in Saudi's foreign exchange reserves, but rounding up 100 princes with demands for ransom certainly could. Regardless, here's the latest report, posted overnight.
Link here:
Maybe later I will do the numbers, but a glance at the chart suggest $2 to $3 billion improvement month-over-month.
Later: from the linked site:
Foreign exchange reserves in Saudi Arabia increased to 1,861,588 SAR million in December from 1,854,104 SAR million in November, 2017.
Foreign exchange reserves in Saudi Arabia averaged 2,213,590 SAR million from 2010 until 2017, reaching an all time high of 2,796,941 SAR million in August, 2014 and a record low of 1,569,145 SAR million in April, 2010.
- Average from 2010 - 2017: 2,213,589,780,000 (USD 598 billion)
- All-time high, August, 2014, 2,796,941,000,000 (USD 755 billion)
Later:
- December, 2017 (latest data available): 1,860,000 million SAR ($502,200,000,000)
- November, 2017: 1,855,000 million SAR ($500,850,000,000)
- Delta: 5,000 million SAR ($1,350,000,000)
So, it appears, Saudi Arabia foreign exchange reserves increased by about USD 1.4 billion, month-over-month.
Disclaimer: I often make simple arithmetic errors, and working with big numbers makes it even more difficult. For all conversions I used the same exchange rate of USD 0.27 for 1 SAR, about a US quarter for each Saudi rial.